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Inquiry is any process that has the aim of augmenting knowledge, resolving doubt, or solving a problem. A theory of inquiry is an account of the various types of inquiry and a treatment of the ways that each type of inquiry achieves its aim.

For lessons on the topic of Inquiry, follow this link.

Classical sources


When three terms are so related to one another that the last is wholly contained in the middle and the middle is wholly contained in or excluded from the first, the extremes must admit of perfect syllogism. By 'middle term' I mean that which both is contained in another and contains another in itself, and which is the middle by its position also; and by 'extremes' (a) that which is contained in another, and (b) that in which another is contained. For if A is predicated of all B, and B of all C, A must necessarily be predicated of all C. … I call this kind of figure the First. (Aristotle, Prior Analytics, 1.4).


Inductive reasoning consists in establishing a relation between one extreme term and the middle term by means of the other extreme; for example, if B is the middle term of A and C, in proving by means of C that A applies to B; for this is how we effect inductions. (Aristotle, Prior Analytics, 2.23).


The locus classicus for the study of abductive reasoning is found in Aristotle's Prior Analytics, Book 2, Chapt. 25. It begins this way: " We have Reduction (απαγωγη, abduction):

  • When it is obvious that the first term applies to the middle, but that the middle applies to the last term is not obvious, yet is nevertheless more probable or not less probable than the conclusion;
  • Or if there are not many intermediate terms between the last and the middle;

For in all such cases the effect is to bring us nearer to knowledge. By way of explanation, Aristotle supplies two very instructive examples, one for each of the two varieties of abductive inference steps that he has just described in the abstract:

  • For example, let A stand for "that which can be taught", B for "knowledge", and C for "morality". Then that knowledge can be taught is evident; but whether virtue is knowledge is not clear. Then if BC is not less probable or is more probable than AC, we have reduction; for we are nearer to knowledge for having introduced an additional term, whereas before we had no knowledge that AC is true.
  • Or again we have reduction if there are not many intermediate terms between B and C; for in this case too we are brought nearer to knowledge. For example, suppose that D is "to square", E "rectilinear figure", and F "circle". Assuming that between E and F there is only one intermediate term — that the circle becomes equal to a rectilinear figure by means of lunules — we should approximate to knowledge.

(Aristotle, "Prior Analytics", 2.25, with minor alterations)

Aristotle's latter variety of abductive reasoning, though it will take some explaining in the sequel, is well worth our contemplation, since it hints already at streams of inquiry that course well beyond the syllogistic source from which they spring, and into regions that Peirce will explore more broadly and deeply.

Inquiry in the pragmatic paradigm

In the pragmatic philosophies of Charles Sanders Peirce, William James, John Dewey, and others, inquiry is closely associated with the normative science of logic. In its inception, the pragmatic model or theory of inquiry was extracted by Peirce from its raw materials in classical logic, with a little bit of help from Kant, and refined in parallel with the early development of symbolic logic by Boole, De Morgan, and Peirce himself to address problems about the nature and conduct of scientific reasoning. Borrowing a brace of concepts from Aristotle, Peirce examined three fundamental modes of reasoning that play a role in inquiry, commonly known as abductive, deductive, and inductive inference.

In rough terms, abduction is what we use to generate a likely hypothesis or an initial diagnosis in response to a phenomenon of interest or a problem of concern, while deduction is used to clarify, to derive, and to explicate the relevant consequences of the selected hypothesis, and induction is used to test the sum of the predictions against the sum of the data. It needs to be observed that the classical and pragmatic treatments of the types of reasoning, dividing the generic territory of inference as they do into three special parts, arrive at a different characterization of the environs of reason than do those accounts that count only two.

These three processes typically operate in a cyclic fashion, systematically operating to reduce the uncertainties and the difficulties that initiated the inquiry in question, and in this way, to the extent that inquiry is successful, leading to an increase in knowledge or in skills.

In the pragmatic way of thinking everything has a purpose, and the purpose of each thing is the first thing we should try to note about it. The purpose of inquiry is to reduce doubt and lead to a state of belief, which a person in that state will usually call knowledge or certainty. As they contribute to the end of inquiry, we should appreciate that the three kinds of inference describe a cycle that can be understood only as a whole, and none of the three makes complete sense in isolation from the others. For instance, the purpose of abduction is to generate guesses of a kind that deduction can explicate and that induction can evaluate. This places a mild but meaningful constraint on the production of hypotheses, since it is not just any wild guess at explanation that submits itself to reason and bows out when defeated in a match with reality. In a similar fashion, each of the other types of inference realizes its purpose only in accord with its proper role in the whole cycle of inquiry. No matter how much it may be necessary to study these processes in abstraction from each other, the integrity of inquiry places strong limitations on the effective modularity of its principal components.

Art and science of inquiry

For our present purposes, the first feature to note in distinguishing the three principal modes of reasoning from each other is whether each of them is exact or approximate in character. In this light, deduction is the only one of the three types of reasoning that can be made exact, in essence, always deriving true conclusions from true premisses, while abduction and induction are unavoidably approximate in their modes of operation, involving elements of fallible judgment in practice and inescapable error in their application.

The reason for this is that deduction, in the ideal limit, can be rendered a purely internal process of the reasoning agent, while the other two modes of reasoning essentially demand a constant interaction with the outside world, a source of phenomena and problems that will no doubt continue to exceed the capacities of any finite resource, human or machine, to master. Situated in this larger reality, approximations can be judged appropriate only in relation to their context of use and can be judged fitting only with regard to a purpose in view.

A parallel distinction that is often made in this connection is to call deduction a demonstrative form of inference, while abduction and induction are classed as non-demonstrative forms of reasoning. Strictly speaking, the latter two modes of reasoning are not properly called inferences at all. They are more like controlled associations of words or ideas that just happen to be successful often enough to be preserved as useful heuristic strategies in the repertoire of the agent. But non-demonstrative ways of thinking are inherently subject to error, and must be constantly checked out and corrected as needed in practice.

In classical terminology, forms of judgment that require attention to the context and the purpose of the judgment are said to involve an element of 'art', in a sense that is judged to distinguish them from 'science', and in their renderings as expressive judgments to implicate arbiters in styles of rhetoric, as contrasted with logic.

In a figurative sense, this means that only deductive logic can be reduced to an exact theoretical science, while the practice of any empirical science will always remain to some degree an art.

Zeroth order inquiry

Many aspects of inquiry can be recognized and usefully studied in very basic logical settings, even simpler than the level of syllogism, for example, in the realm of reasoning that is variously known as Boolean algebra, propositional calculus, sentential calculus, or zeroth-order logic. By way of approaching the learning curve on the gentlest availing slope, we may well begin at the level of zeroth-order inquiry, in effect, taking the syllogistic approach to inquiry only so far as the propositional or sentential aspects of the associated reasoning processes are concerned. One of the bonuses of doing this in the context of Peirce's logical work is that it provides us with doubly instructive exercises in the use of his logical graphs, taken at the level of his so-called 'alpha graphs'.

In the case of propositional calculus or sentential logic, deduction comes down to applications of the transitive law for conditional implications and the approximate forms of inference hang on the properties that derive from these. In describing the various types of inference I will employ a few old 'terms of art' from classical logic that are still of use in treating these kinds of simple problems in reasoning.

Deduction takes a Case, the minor premiss <math>X \Rightarrow Y</math>
and combines it with a Rule,the major premiss <math>Y \Rightarrow Z</math>
to arrive at a Fact, the demonstrative conclusion <math>X \Rightarrow Z.</math>
Induction takes a Case of the form <math>X \Rightarrow Y</math>
and matches it with a Fact of the form <math>X \Rightarrow Z</math>
to infer a Rule of the form <math>Y \Rightarrow Z.</math>
Abduction takes a Fact of the form <math>X \Rightarrow Z</math>
and matches it with a Rule of the form <math>Y \Rightarrow Z</math>
to infer a Case of the form <math>X \Rightarrow Y.</math>

For ease of reference, Figure 1 and the Legend beneath it summarize the classical terminology for the three types of inference and the relationships among them.

|                                                 |
|                   Z                             |
|                   o                             |
|                   |\                            |
|                   | \                           |
|                   |  \                          |
|                   |   \                         |
|                   |    \                        |
|                   |     \   R U L E             |
|                   |      \                      |
|                   |       \                     |
|               F   |        \                    |
|                   |         \                   |
|               A   |          \                  |
|                   |           o Y               |
|               C   |          /                  |
|                   |         /                   |
|               T   |        /                    |
|                   |       /                     |
|                   |      /                      |
|                   |     /   C A S E             |
|                   |    /                        |
|                   |   /                         |
|                   |  /                          |
|                   | /                           |
|                   |/                            |
|                   o                             |
|                   X                             |
|                                                 |
| Deduction takes a Case of the form X => Y,      |
| matches it with a Rule of the form Y => Z,      |
| then adverts to a Fact of the form X => Z.      |
|                                                 |
| Induction takes a Case of the form X => Y,      |
| matches it with a Fact of the form X => Z,      |
| then adverts to a Rule of the form Y => Z.      |
|                                                 |
| Abduction takes a Fact of the form X => Z,      |
| matches it with a Rule of the form Y => Z,      |
| then adverts to a Case of the form X => Y.      |
|                                                 |
| Even more succinctly:                           |
|                                                 |
|           Abduction  Deduction  Induction       |
|                                                 |
| Premiss:     Fact       Rule       Case         |
| Premiss:     Rule       Case       Fact         |
| Outcome:     Case       Fact       Rule         |
|                                                 |
Figure 1.  Elementary Structure and Terminology

In its original usage a statement of Fact has to do with a deed done or a record made, that is, a type of event that is openly observable and not riddled with speculation as to its very occurrence. In contrast, a statement of Case may refer to a hidden or a hypothetical cause, that is, a type of event that is not immediately observable to all concerned. Obviously, the distinction is a rough one and the question of which mode applies can depend on the points of view that different observers adopt over time. Finally, a statement of a Rule is called that because it states a regularity or a regulation that governs a whole class of situations, and not because of its syntactic form. So far in this discussion, all three types of constraint are expressed in the form of conditional propositions, but this is not a fixed requirement. In practice, these modes of statement are distinguished by the roles that they play within an argument, not by their style of expression. When the time comes to branch out from the syllogistic framework, we will find that propositional constraints can be discovered and represented in arbitrary syntactic forms.

Kinds of inference

The three kinds of inference that Peirce would come to refer to as abductive, deductive, and inductive inference he gives his earliest systematic treatment in two series of lectures on the logic of science: the Harvard University Lectures of 1865 and the Lowell Institute Lectures of 1866. There he sums up the characters of the three kinds of reasoning in the following terms:

  • We have then three different kinds of inference:
Deduction or inference à priori,
Induction or inference à particularis, and
Hypothesis or inference à posteriori.
(Peirce, "On the Logic of Science" (1865), W 1, 267).

Early in the first series of lectures Peirce gives a very revealing illustration of how he then thinks of the natures, operations, and relationships of this trio of inference types:

  • If I reason that certain conduct is wise
because it has a character which belongs
only to wise things, I reason à priori.
  • If I think it is wise because it once turned out
to be wise, that is, if I infer that it is wise on
this occasion because it was wise on that occasion,
I reason inductively [à particularis].
  • But if I think it is wise because a wise man does it,
I then make the pure hypothesis that he does it
because he is wise, and I reason à posteriori.
(Peirce, "On the Logic of Science" (1865), W 1, 180).

We may begin the analysis of Peirce's example by making the following assignments of letters to the qualitative attributes mentioned in it:

  • A = 'Wisdom',
  • B = 'a certain character',
  • C = 'a certain conduct',
  • D = 'done by a wise man',
  • E = 'a certain occasion'.

Recognizing that a little more concreteness will serve as an aid to the understanding, let's augment the Spartan features of Peirce's illustration in the following way:

  • B = 'Benevolence', a certain character,
  • C = 'Contributes to Charity', a certain conduct,
  • E = 'Earlier today', a certain occasion.

The converging operation of all three reasonings is shown in Figure 2.

|                                                                     |
|  D ("done by a wise man")                                           |
|   o                                                                 |
|    \*                                                               |
|     \ *                                                             |
|      \  *                                                           |
|       \   *                                                         |
|        \    *                                                       |
|         \     *                                                     |
|          \      * A ("a wise act")                                  |
|           \       o                                                 |
|            \     /| *                                               |
|             \   / |   *                                             |
|              \ /  |     *                                           |
|               .   |       o B ("benevolence", a certain character)  |
|              / \  |     *                                           |
|             /   \ |   *                                             |
|            /     \| *                                               |
|           /       o                                                 |
|          /      * C ("contributes to charity", a certain conduct)   |
|         /     *                                                     |
|        /    *                                                       |
|       /   *                                                         |
|      /  *                                                           |
|     / *                                                             |
|    /*                                                               |
|   o                                                                 |
|  E ("earlier today", a certain occasion)                            |
|                                                                     |
Figure 2.  A Thrice Wise Act

One of the styles of syntax that Aristotle uses for syllogistic propositions suggests the composite symbols that geometers have long used for labeling line intervals in a geometric figure, and it comports quite nicely with the Figure that we have just drawn. Specifically, the proposition that predicates X of the subject Y is represented by the digram 'XY' and associated with the line interval XY that descends from the point X to the point Y in the corresponding lattice diagram. In this wise we make the following observations:

The common proposition that concludes each argument is AC. Introducing the symbol '=>' to denote the relation of logical implication, the proposition AC can be written as C => A, and read as 'C implies A'. Adopting the parenthetical form of Peirce's alpha graphs, in their 'existential interpretation', AC can be written as (C (A)), and most easily comprehended as 'not C without A'. In the context of the present example, all of these forms are equally good ways of expressing the same concrete proposition, namely, 'contributing to charity is wise'.

  • Deduction could have obtained the Fact AC from the Rule AB, 'benevolence is wisdom', along with the Case BC, 'contributing to charity is benevolent'.
  • Induction could have gathered the Rule AC, after a manner of saying that 'contributing to charity is exemplary of wisdom', from the Fact AE, 'the act of earlier today is wise', along with the Case CE, 'the act of earlier today was an instance of contributing to charity'.
  • Abduction could have guessed the Case AC, in a style of expression stating that 'contributing to charity is explained by wisdom', from the Fact DC, 'contributing to charity is done by this wise man', and the Rule DA, 'everything that is wise is done by this wise man'. Thus, a wise man, who happens to do all of the wise things that there are to do, may nevertheless contribute to charity for no good reason, and even be known to be charitable to a fault. But all of this notwithstanding, on seeing the wise man contribute to charity we may find it natural to conjecture, in effect, to consider it as a possibility worth examining further, that charity is indeed a mark of his wisdom, and not just the accidental trait or the immaterial peculiarity of his character — in essence, that wisdom is the 'cause' of his contribution or the 'reason' for his charity.

As a general rule, and despite many obvious exceptions, an English word that ends in '-ion' denotes equivocally either a process or its result. In our present application, this means that each of the words 'abduction', 'deduction', 'induction' can be used to denote either the process of inference or the product of that inference, that is, the proposition to which the inference in question leads.

One of the morals of Peirce's illustration can now be drawn. It demonstrates in a very graphic fashion that the three kinds of inference are three kinds of process and not three kinds of proposition, not if one takes the word 'kind' in its literal sense as denoting a genus of being, essence, or substance. Said another way, it means that being an abductive Case, a deductive Fact, or an inductive Rule is a category of relation, indeed, one that involves at the very least a triadic relation among propositions, and not a category of essence or substance, that is, not a property that inheres in the proposition alone.

This category distinction between the absolute, essential, or monadic predicates and the more properly relative predicates constitutes a very important theme in Peirce's architectonic. There is of course a parallel application of it in the theory of sign relations, or semiotics, where the distinctions among the sign relational roles of Object, Sign, and Interpretant are distinct ways of relating to other things, modes of relation that may vary from moment to moment in the extended trajectory of a sign process, and not distinctions that mark some fixed and eternal essence of the thing in itself.

In the normal course of inquiry, the elementary types of inference proceed in the order: Abduction, Deduction, Induction. However, the same building blocks can be assembled in other ways to yield different types of complex inferences. Of particular importance, reasoning by analogy can be analyzed as a combination of induction and deduction, in other words, as the abstraction and the application of a rule. Because a complicated pattern of analogical inference will be used in our example of a complete inquiry, it will help to prepare the ground if we first stop to consider an example of analogy in its simplest form.


Much of Peirce's work deals with the scientific and logical questions of knowledge and truth, questions grounded in his experience as a working logician and experimental scientist, one who was a member of the international community of scientists and thinkers of his day. He made important contributions to deductive logic (see below), but was primarily interested in the logic of science and specifically in what he called abduction or "hypothesis", as opposed to deduction and induction. Abduction is the process whereby a hypothesis is generated, so that surprising facts may be explained. "There is a more familiar name for it than abduction", Peirce wrote, "for it is neither more nor less than guessing". Indeed, Peirce considered abduction to be at the heart not only of scientific research but of native human intelligence as well.

In his "Illustrations of the Logic of Science" (W 3, 325-326), Peirce gives the following example of how abduction nests with deductive and inductive reasoning. Peirce begins by positing the following three statements:

  • Rule: "All the beans from this bag are white."
  • Case: "These beans are from this bag."
  • Result: "These beans are white."

Now let any two of these statements be Givens (their order not mattering), and let the remaining statement be the Conclusion. The result is an argument, of which three kinds are possible:

  Deduction Induction Abduction
Given Rule Case Rule
Given Case Result Result
Conclusion Result Rule Case

Deduction encompasses, of course, the classical syllogism.


The classic description of analogy in the syllogistic frame comes from Aristotle, who called this form of inference by the name paradeigma, that is, reasoning by way of example or through the parallel comparison of cases.

We have an Example [παραδειγμα, analogy] when the major extreme is shown to be applicable to the middle term by means of a term similar to the third. It must be known both that the middle applies to the third term and that the first applies to the term similar to the third. (Aristotle, "Prior Analytics", 2.24).

Aristotle illustrates this pattern of argument with the following sample of reasoning. The setting is a discussion, taking place in Athens, on the issue of going to war with Thebes. It is apparently accepted that a war between Thebes and Phocis is or was a bad thing, perhaps from the objectivity lent by non-involvement or perhaps as a lesson of history.

For example, let A be 'bad', B 'to make war on neighbors', C 'Athens against Thebes', and D 'Thebes against Phocis'. Then if we require to prove that war against Thebes is bad, we must be satisfied that war against neighbors is bad. Evidence of this can be drawn from similar examples, for example, that war by Thebes against Phocis is bad. Then since war against neighbors is bad, and war against Thebes is war against neighbors, it is evident that war against Thebes is bad.(Aristotle, "Prior Analytics", 2.24, with minor alterations).

Aristotle's sample of argument from analogy may be analyzed in the following way:

First, a Rule is induced from the consideration of a similar Case and a relevant Fact:

  • Case: D => B, Thebes vs Phocis is war against neighbors.
  • Fact: D => A, Thebes vs Phocis is bad.
  • Rule: B => A, War against neighbors is bad.

Next, the Fact to be proved is deduced from the application of the previously induced Rule to the present Case:

  • Case: C => B, Athens vs Thebes is war against neighbors.
  • Rule: B => A, War against neighbors is bad.
  • Fact: C => A, Athens vs Thebes is bad.

In practice, of course, it would probably take a mass of comparable cases to establish a rule. As far as the logical structure goes, however, this quantitative confirmation only amounts to 'gilding the lily'. Perfectly valid rules can be guessed on the first try, abstracted from a single experience or adopted vicariously with no personal experience. Numerical factors only modify the degree of confidence and the strength of habit that govern the application of previously learned rules.

Figure 3 gives a graphical illustration of Aristotle's example of 'Example', that is, the form of reasoning that proceeds by Analogy or according to a Paradigm.

|                                                           |
|                             A                             |
|                             o                             |
|                            /*\                            |
|                           / * \                           |
|                          /  *  \                          |
|                         /   *   \                         |
|                        /    *    \                        |
|                       /     *     \                       |
|                      /   R u l e   \                      |
|                     /       *       \                     |
|                    /        *        \                    |
|                   /         *         \                   |
|                  /          *          \                  |
|              F a c t        o        F a c t              |
|                /          * B *          \                |
|               /         *       *         \               |
|              /        *           *        \              |
|             /       *               *       \             |
|            /   C a s e            C a s e    \            |
|           /     *                       *     \           |
|          /    *                           *    \          |
|         /   *                               *   \         |
|        /  *                                   *  \        |
|       / *                                       * \       |
|      o                                             o      |
|     C                                               D     |
|                                                           |
| A  =  Atrocious, Adverse to All, A bad thing              |
| B  =  Belligerent Battle Between Brethren                 |
| C  =  Contest of Athens against Thebes                    |
| D  =  Debacle of Thebes against Phocis                    |
|                                                           |
| A is a major term                                         |
| B is a middle term                                        |
| C is a minor term                                         |
| D is a minor term, similar to C                           |
|                                                           |
Figure 3.  Aristotle's 'War Against Neighbors' Example

In this analysis of reasoning by Analogy, it is a complex or a mixed form of inference that can be seen as taking place in two steps:

  • The first step is an Induction that abstracts a Rule from a Case and a Fact.
Case: D => B, Thebes vs Phocis is a battle between neighbors.
Fact: D => A, Thebes vs Phocis is adverse to all.
Rule: B => A, A battle between neighbors is adverse to all.
  • The final step is a Deduction that applies this Rule to a Case to arrive at a Fact.
Case: C => B, Athens vs Thebes is a battle between neighbors.
Rule: B => A, A battle between neighbors is adverse to all.
Fact: C => A, Athens vs Thebes is adverse to all.

As we see, Aristotle analyzed analogical reasoning into a phase of inductive reasoning followed by a phase of deductive reasoning. Peirce would pick up the story at this juncture and eventually parse analogy in a couple of different ways, both of them involving all three types of inference: abductive, deductive, and inductive.

Example of inquiry

Examples of inquiry, that illustrate the full cycle of its abductive, deductive, and inductive phases, and yet are both concrete and simple enough to be suitable for a first (or zeroth) exposition, are somewhat rare in Peirce's writings, and so let us draw one from the work of fellow pragmatician John Dewey, analyzing it according to the model of zeroth-order inquiry that we developed above.

A man is walking on a warm day. The sky was clear the last time he observed it; but presently he notes, while occupied primarily with other things, that the air is cooler. It occurs to him that it is probably going to rain; looking up, he sees a dark cloud between him and the sun, and he then quickens his steps. What, if anything, in such a situation can be called thought? Neither the act of walking nor the noting of the cold is a thought. Walking is one direction of activity; looking and noting are other modes of activity. The likelihood that it will rain is, however, something suggested. The pedestrian feels the cold; he thinks of clouds and a coming shower. (John Dewey, How We Think, pp. 6-7).

Once over quickly

Let's first give Dewey's elegant example of inquiry in everyday life the quick once over, hitting just the high points of its analysis into Peirce's three kinds of reasoning.

Abductive phase

In Dewey's 'Rainy Day' or 'Sign of Rain' story, we find our peripatetic hero presented with a surprising Fact:

  • Fact: C => A, In the Current situation the Air is cool.

Responding to an intellectual reflex of puzzlement about the situation, his resource of common knowledge about the world is impelled to seize on an approximate Rule:

  • Rule: B => A, Just Before it rains, the Air is cool.

This Rule can be recognized as having a potential relevance to the situation because it matches the surprising Fact, C => A, in its consequential feature A.

All of this suggests that the present Case may be one in which it is just about to rain:

  • Case: C => B, The Current situation is just Before it rains.

The whole mental performance, however automatic and semi-conscious it may be, that leads up from a problematic Fact and a previously settled knowledge base of Rules to the plausible suggestion of a Case description, is what we are calling an abductive inference.

Deductive phase

The next phase of inquiry uses deductive inference to expand the implied consequences of the abductive hypothesis, with the aim of testing its truth. For this purpose, the inquirer needs to think of other things that would follow from the consequence of his precipitate explanation. Thus, he now reflects on the Case just assumed:

  • Case: C => B, The Current situation is just Before it rains.

He looks up to scan the sky, perhaps in a random search for further information, but since the sky is a logical place to look for details of an imminent rainstorm, symbolized in our story by the letter B, we may safely suppose that our reasoner has already detached the consequence of the abduced Case, C => B, and has begun to expand on its further implications. So let us imagine that our up-looker has a more deliberate purpose in mind, and that his search for additional data is driven by the new-found, determinate Rule:

  • Rule: B => D, Just Before it rains, Dark clouds appear.

Contemplating the assumed Case in combination with this new Rule leads him by an immediate deduction to predict an additional Fact:

  • Fact: C => D, In the Current situation Dark clouds appear.

The reconstructed picture of reasoning assembled in this second phase of inquiry is true to the pattern of deductive inference.

Inductive phase

Whatever the case, our subject observes a Dark cloud, just as he would expect on the basis of the new hypothesis. The explanation of imminent rain removes the discrepancy between observations and expectations and thereby reduces the shock of surprise that made this process of inquiry necessary.

Looking more closely

Seeding hypotheses

Figure 4 gives a graphical illustration of Dewey's example of inquiry, isolating for the purposes of the present analysis the first two steps in the more extended proceedings that go to make up the whole inquiry.

|                                                           |
|     A                                               D     |
|      o                                             o      |
|       \ *                                       * /       |
|        \  *                                   *  /        |
|         \   *                               *   /         |
|          \    *                           *    /          |
|           \     *                       *     /           |
|            \   R u l e             R u l e   /            |
|             \       *               *       /             |
|              \        *           *        /              |
|               \         *       *         /               |
|                \          * B *          /                |
|              F a c t        o        F a c t              |
|                  \          *          /                  |
|                   \         *         /                   |
|                    \        *        /                    |
|                     \       *       /                     |
|                      \   C a s e   /                      |
|                       \     *     /                       |
|                        \    *    /                        |
|                         \   *   /                         |
|                          \  *  /                          |
|                           \ * /                           |
|                            \*/                            |
|                             o                             |
|                             C                             |
|                                                           |
| A  =  the Air is cool                                     |
| B  =  just Before it rains                                |
| C  =  the Current situation                               |
| D  =  a Dark cloud appears                                |
|                                                           |
| A is a major term                                         |
| B is a middle term                                        |
| C is a minor term                                         |
| D is a major term, associated with A                      |
|                                                           |
Figure 4.  Dewey's 'Rainy Day' Inquiry

In this analysis of the first steps of Inquiry, we have a complex or a mixed form of inference that can be seen as taking place in two steps:

  • The first step is an Abduction that abstracts a Case from the consideration of a Fact and a Rule.
Fact: C => A, In the Current situation the Air is cool.
Rule: B => A, Just Before it rains, the Air is cool.
Case: C => B, The Current situation is just Before it rains.
  • The final step is a Deduction that admits this Case to another Rule and so arrives at a novel Fact.
Case: C => B, The Current situation is just Before it rains.
Rule: B => D, Just Before it rains, a Dark cloud will appear.
Fact: C => D, In the Current situation, a Dark cloud will appear.

This is nowhere near a complete analysis of the Rainy Day inquiry, even insofar as it might be carried out within the constraints of the syllogistic framework, and it covers only the first two steps of the relevant inquiry process, but maybe it will do for a start.

One other thing needs to be noticed here, the formal duality between this expansion phase of inquiry and the argument from analogy. This can be seen most clearly in the propositional lattice diagrams shown in Figures 3 and 4, where analogy exhibits a rough "A" shape and the first two steps of inquiry exhibit a rough "V" shape, respectively. Since we find ourselves repeatedly referring to this expansion phase of inquiry as a unit, let's give it a name that suggests its duality with analogy — 'catalogy' will do for the moment. This usage is apt enough if one thinks of a catalogue entry for an item as a text that lists its salient features. Notice that analogy has to do with the examples of a given quality, while catalogy has to do with the qualities of a given example. Peirce noted similar forms of duality in many of his early writings, leading to the consummate treatment in his 1867 paper "On a New List of Categories" (CP 1.545-559, W 2, 49-59).

Weeding hypotheses

In order to comprehend the bearing of inductive reasoning on the closing phases of inquiry there are a couple of observations that we need to make:

  • First, we need to recognize that smaller inquiries are typically woven into larger inquiries, whether we view the whole pattern of inquiry as carried on by a single agent or by a complex community.
  • Further, we need to consider the different ways in which the particular instances of inquiry can be related to ongoing inquiries at larger scales. Three modes of inductive interaction between the micro-inquiries and the macro-inquiries that are salient here can be described under the headings of the 'Learning', the 'Transfer', and the 'Testing' of rules.

Analogy of experience

Throughout inquiry the reasoner makes use of rules that have to be transported across intervals of experience, from the masses of experience where they are learned to the moments of experience where they are applied. Inductive reasoning is involved in the learning and the transfer of these rules, both in accumulating a knowledge base and in carrying it through the times between acquisition and application.

  • Learning. The principal way that induction contributes to an ongoing inquiry is through the learning of rules, that is, by creating each of the rules that goes into the knowledge base, or ever gets used along the way.
  • Transfer. The continuing way that induction contributes to an ongoing inquiry is through the exploit of analogy, a two-step combination of induction and deduction that serves to transfer rules from one context to another.
  • Testing. Finally, every inquiry that makes use of a knowledge base constitutes a 'field test' of its accumulated contents. If the knowledge base fails to serve any live inquiry in a satisfactory manner, then there is a prima facie reason to reconsider and possibly to amend some of its rules.

Let's now consider how these principles of learning, transfer, and testing apply to John Dewey's 'Sign of Rain' example.


Rules in a knowledge base, as far as their effective content goes, can be obtained by any mode of inference.

For example, a rule like:

  • Rule: B => A, Just Before it rains, the Air is cool,

is usually induced from a consideration of many past events, in a manner that can be rationally reconstructed as follows:

  • Case: C => B, In Certain events, it is just Before it rains,
  • Fact: C => A, In Certain events, the Air is cool,
  • Rule: B => A, Just Before it rains, the Air is cool.

However, the very same proposition could also be abduced as an explanation of a singular occurrence or deduced as a conclusion of a presumptive theory.


What is it that gives a distinctively inductive character to the acquisition of a knowledge base? It is evidently the 'analogy of experience' that underlies its useful application. Whenever we find ourselves prefacing an argument with the phrase 'If past experience is any guide …' then we can be sure that this principle has come into play. We are invoking an analogy between past experience, considered as a totality, and present experience, considered as a point of application. What we mean in practice is this: 'If past experience is a fair sample of possible experience, then the knowledge gained in it applies to present experience'. This is the mechanism that allows a knowledge base to be carried across gulfs of experience that are indifferent to the effective contents of its rules.

Here are the details of how this notion of transfer works out in the case of the 'Sign of Rain' example:

Let K(pres) be a portion of the reasoner's knowledge base that is logically equivalent to the conjunction of two rules, as follows:

  • K(pres) = (B => A) and (B => D).

K(pres) is the present knowledge base, expressed in the form of a logical constraint on the present universe of discourse.

It is convenient to have the option of expressing all logical statements in terms of their logical models, that is, in terms of the primitive circumstances or the elements of experience over which they hold true.

  • Let E(past) be the chosen set of experiences, or the circumstances that we have in mind when we refer to 'past experience'.
  • Let E(poss) be the collective set of experiences, or the projective total of possible circumstances.
  • Let E(pres) be the present experience, or the circumstances that are present to the reasoner at the current moment.

If we think of the knowledge base K(pres) as referring to the 'regime of experience' over which it is valid, then all of these sets of models can be compared by the simple relations of set inclusion or logical implication.

Figure 5 schematizes this way of viewing the 'analogy of experience'.

|                                                           |
|                          K(pres)                          |
|                             o                             |
|                            /|\                            |
|                           / | \                           |
|                          /  |  \                          |
|                         /   |   \                         |
|                        /  Rule   \                        |
|                       /     |     \                       |
|                      /      |      \                      |
|                     /       |       \                     |
|                    /     E(poss)     \                    |
|              Fact /         o         \ Fact              |
|                  /        *   *        \                  |
|                 /       *       *       \                 |
|                /      *           *      \                |
|               /     *               *     \               |
|              /    *                   *    \              |
|             /   *  Case           Case  *   \             |
|            /  *                           *  \            |
|           / *                               * \           |
|          /*                                   *\          |
|         o<<<---------------<<<---------------<<<o         |
|      E(past)        Analogy Morphism         E(pres)      |
|    More Known                              Less Known     |
|                                                           |
Figure 5.  Analogy of Experience

In these terms, the 'analogy of experience' proceeds by inducing a Rule about the validity of a current knowledge base and then deducing a Fact, its applicability to a current experience, as in the following sequence:

Inductive Phase:

  • Given Case: E(past) => E(poss), Chosen events fairly sample Collective events.
  • Given Fact: E(past) => K(pres), Chosen events support the Knowledge regime.
  • Induce Rule: E(poss) => K(pres), Collective events support the Knowledge regime.

Deductive Phase:

  • Given Case: E(pres) => E(poss), Current events fairly sample Collective events.
  • Given Rule: E(poss) => K(pres), Collective events support the Knowledge regime.
  • Deduce Fact: E(pres) => K(pres), Current events support the Knowledge regime.

If the observer looks up and does not see dark clouds, or if he runs for shelter but it does not rain, then there is fresh occasion to question the utility or the validity of his knowledge base. But we must leave our foulweather friend for now and defer the logical analysis of this testing phase to another occasion.


"Animals respond nobly to the urge of life, but only man can attain the art of living, albeit the majority of mankind only experience the animal urge to live. Animals know only this blind and instinctive urge; man is capable of transcending this urge to natural function. Man may elect to live upon the high plane of intelligent art, even that of celestial joy and spiritual ecstasy. Animals make no inquiry into the purposes of life; therefore they never worry, neither do they commit suicide. Suicide among men testifies that such beings have emerged from the purely animal stage of existence, and to the further fact that the exploratory efforts of such human beings have failed to attain the artistic levels of mortal experience. Animals know not the meaning of life; man not only possesses capacity for the recognition of values and the comprehension of meanings, but he also is conscious of the meaning of meanings--he is self-conscious of insight." [1]


  • Angluin, Dana (1989), "Learning with Hints", pp. 167–181 in David Haussler and Leonard Pitt (eds.), Proceedings of the 1988 Workshop on Computational Learning Theory, MIT, 3–5 August 1988, Morgan Kaufmann, San Mateo, CA, 1989.
  • Awbrey, Jon, and Awbrey, Susan (1995), "Interpretation as Action : The Risk of Inquiry", Inquiry : Critical Thinking Across the Disciplines 15, 40–52. Eprint.
  • Delaney, C.F. (1993), Science, Knowledge, and Mind: A Study in the Philosophy of C.S. Peirce, University of Notre Dame Press, Notre Dame, IN.
  • Dewey, John (1910), How We Think, D.C. Heath, Lexington, MA, 1910. Reprinted, Prometheus Books, Buffalo, NY, 1991.
  • Dewey, John (1938), Logic: The Theory of Inquiry, Henry Holt and Company, New York, NY, 1938. Reprinted as pp. 1–527 in John Dewey, The Later Works, 1925–1953, Volume 12: 1938, Jo Ann Boydston (ed.), Kathleen Poulos (text. ed.), Ernest Nagel (intro.), Southern Illinois University Press, Carbondale and Edwardsville, IL, 1986.
  • Haack, Susan (1993), Evidence and Inquiry: Towards Reconstruction in Epistemology, Blackwell Publishers, Oxford, UK.
  • Hanson, Norwood Russell (1958), Patterns of Discovery, An Inquiry into the Conceptual Foundations of Science, Cambridge University Press, Cambridge, UK.
  • Misak, Cheryl J. (1991), Truth and the End of Inquiry, A Peircean Account of Truth, Oxford University Press, Oxford, UK.

See also

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The Art of the Question