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[[Act]] is, therefore, the primal sense. Potency is always said in [[reference]] to it. Accordingly, the different senses of act, which Aristotle also recognizes, must correspond to different senses of potency.

[[Act]] is, therefore, the primal sense. Potency is always said in [[reference]] to it. Accordingly, the different senses of act, which Aristotle also recognizes, must correspond to different senses of potency.

==Senses of act==

==Senses of act==

According to Aristotle, there are three great senses of act: [[movement]] (kinesis), [[form]], and [[knowledge]]. Other senses, like [[nature]] (phusis), [[habit]] (hexis), etc., may be reduced to these three. Movement is defined by Aristotle in his book on Physics as "The [[fulfillment]] of what exists potentially, insofar as it exists potentially, is [[motion]]". This is known as a ''transitive'' act. Transitivity is the [[expression]] of an act in which something [[changes]] for something else. Movement can therefore be understood under the more general term of metabolé, that is, change. The corresponding potency is, thus, a transitive potency, or kinetic potency.

According to Aristotle, there are three great senses of act: [[movement]] (kinesis), [http://en.wikipedia.org/wiki/Theory_of_Forms form], and [[knowledge]]. Other senses, like [[nature]] (phusis), [[habit]] (hexis), etc., may be reduced to these three. Movement is defined by Aristotle in his book on Physics as "The [[fulfillment]] of what exists potentially, insofar as it exists potentially, is [[motion]]". This is known as a ''transitive'' act. Transitivity is the [[expression]] of an act in which something [[changes]] for something else. Movement can therefore be understood under the more general term of metabolé, that is, change. The corresponding potency is, thus, a transitive potency, or kinetic potency.

Now, substance is a [[formal]] act. Moreover, a formal act is always the end of a transitive movement. Substance can also then be understood as metabolé. Indeed, as Aristotle explains movement, it always implies: a cause from which it takes place, a subject in which it takes place, and a form (end, péras) it which knowledge takes place. In all these cases, movement is always found within the categories (the second sense of being mentioned above) and is always a [[Causality|causal]] act.

Now, substance is a [[formal]] act. Moreover, a formal act is always the end of a transitive movement. Substance can also then be understood as metabolé. Indeed, as Aristotle explains movement, it always implies: a cause from which it takes place, a subject in which it takes place, and a form (end, péras) it which knowledge takes place. In all these cases, movement is always found within the categories (the second sense of being mentioned above) and is always a [[Causality|causal]] act.

In the ''Metaphysics'' Aristotle gives extraordinary examples: we see and we have seen, we think and have thought; but we don’t build and have built. Building is a transitive action, an imperfect action. In the [[perfect]] action, ''praxis'', the object (the end of the action) is immediately possessed, at the same time, [[simultaneously]]. Knowledge is not a change of a form, a change of [[quality]], because a form is not corrupted by the acquisition of a new one; rather, both forms co-exist in one act. That is why knowledge is not only the possession of a form: it is the possession of another being’s form. And not only that: knowledge is the possession of another’s form as another’s form. When knowing a stone, we do not become the stone, even as we do have its form as one in act with the act of knowledge. Movement does not possess its object. Movement does not exist at once with its object, for possessing the object would mean the end of movement. Movement is not co-actual with its end, as knowledge is.

In the ''Metaphysics'' Aristotle gives extraordinary examples: we see and we have seen, we think and have thought; but we don’t build and have built. Building is a transitive action, an imperfect action. In the [[perfect]] action, ''praxis'', the object (the end of the action) is immediately possessed, at the same time, [[simultaneously]]. Knowledge is not a change of a form, a change of [[quality]], because a form is not corrupted by the acquisition of a new one; rather, both forms co-exist in one act. That is why knowledge is not only the possession of a form: it is the possession of another being’s form. And not only that: knowledge is the possession of another’s form as another’s form. When knowing a stone, we do not become the stone, even as we do have its form as one in act with the act of knowledge. Movement does not possess its object. Movement does not exist at once with its object, for possessing the object would mean the end of movement. Movement is not co-actual with its end, as knowledge is.

==Potency and possibility==

==Potency and possibility==

According to what we have said, if knowledge is not a transitive action, its correspondent potency should also be intransitive. Indeed, it is possible in some [[texts]] to find an account of possibility which suggests that Aristotle has in [[mind]] a different form of potency. However, the texts are not clear enough, and many [[interpretations]] have been given to them. One could, for example, read the following passages: ''Metaphysics IX, 3, Bk 1047a 24'';IX, 6, Bk 1048a 28-30; ''De Interpretatione 23a 8-16''. In this last text Aristotle states that this potency corresponds not only to [[Temporal|mobile]] [[being]], but also to [[eternal]] beings (that is, beings outside the range of kinesis).

According to what we have said, if knowledge is not a transitive action, its correspondent potency should also be intransitive. Indeed, it is possible in some [[texts]] to find an account of possibility which suggests that Aristotle has in [[mind]] a different form of potency. However, the texts are not clear enough, and many [[interpretations]] have been given to them. One could, for example, read the following passages: ''Metaphysics IX, 3, Bk 1047a 24'';IX, 6, Bk 1048a 28-30; ''De Interpretatione 23a 8-16''. In this last text Aristotle states that this potency corresponds not only to [[Temporal|mobile]] [[being]], but also to [[eternal]] beings (that is, beings outside the range of kinesis).