German philosophy, here taken to mean either (1) philosophy in the German language or (2) philosophy by Germans, has been extremely diverse, and central to both the analytic and continental traditions in philosophy for centuries, from Leibniz through Kant, Hegel, Marx, Schopenhauer, Nietzsche, Wittgenstein, to contemporary philosophers.
In the 18th century, Immanuel Kant published his Critique of Pure Reason, in which he attempted to determine what we can and cannot know through the use of reason independent of all experience. Briefly, he came to the conclusion that we could come to know an external world through experience, but that what we could know about it was limited by the limited terms in which the mind can think: if we can only comprehend things in terms of cause and effect, then we can only know causes and effects. It follows from this that we can know the form of all possible experience independent of all experience, but nothing else, but we can never know the world from the “standpoint of nowhere” and therefore we can never know the world in its entirety, neither via reason nor experience.
Since the publication of his Critique, Immanuel Kant has been considered one of the greatest influences in all of western philosophy. In the late 18th and early 19th century, one direct line of influence from Kant is German Idealism.
The German Idealists believed there were problems with Kant’s system and sought to place it on firmer grounds. They were also greatly concerned with the problem of freewill as understood through Kantianism: practical reason presupposes a freewill, and yet according to theoretical reason, everything is predetermined in a complete system of causality. Therefore either everything in possible experience isn’t predetermined, which contradicts the universality of pure reason, or the freewill is outside the system of causality and can have no effect on it, rendering the will useless.
The three most prominent German Idealists were Fichte, Schelling and Hegel. On some interpretations, Hegel did away with Kantianism altogether to achieve absolute knowledge, while others read him as working within the confines of Kantianism. Either way, he came to replace Kant as the dominant influence in German Philosophy for the rest of the 19th century, and his method of dialectics has become a commonplace means of reasoning in continental philosophy.
Karl Marx and The Young Hegelians
Among those influenced by Hegel was a group of young radicals called the Young Hegelians of the 19th century, who were unpopular because of their radical views on religion and society. They included Ludwig Feuerbach, Bruno Bauer and Max Stirner among their ranks.
Karl Marx often attended the groups meetings. He developed an interest in Hegelianism, French socialism and British Economic Theory. He transformed the three into an essential work of economics called Das Kapital, which consisted of a critical economic examination of capitalism. Marxism has had a massive affect on the world as a whole.
In the mid 19th century, Friedrich Nietzsche was a proponent of the German philosopher Arthur Schopenhauer. However, he soon came to disavowal his pessimistic outlook on life and sought to provide a positive philosophy. He believed this task to be urgent, as he believed a form of nihilism caused by modernity was spreading across Europe, which he summed up in the phrase "God is dead". His problem, then, was how to live a positive life considering the fact that if you believe in God, you give into nihilism, and if you don't believe in God, you also give in to nihilism. He believed he found his solution in the concepts of the Overman and Eternal Return. His work continues to have a major influence on both philosophers and artists.
The Vienna Circle, Frege and Wittgenstein
In the early part of the 20th century, a young group of German Philosophers formed the Vienna Circle to promote scientific thought over Hegelian system-building, which they saw as a bad influence on intellectual thought. The group was the beginning of analytic philosophy. They considered themselves logical positivists because they believed all knowledge is either derived through experience or arrived at through analytic statements, and they adopted the predicate logic of Gottlob Frege, which overthrew Aristotelian logic (the dominant logic since its inception in Ancient Greece), and the early work of Wittgenstein as foundations to their work. Wittgenstein did not agree with their interpretation of his philosophy.
Phenomenology began in the early 20th century with the descriptive psychology of Franz Brentano, and then the transcendental phenomenology of Edmund Husserl. It was then transformed by Martin Heidegger, who, along with Ludwig Wittgenstein, is considered one of the most influential philosophers of the 20th century. Phenomenology has had a large influence on Continental Philosophy, particularly existentialism and poststructuralism.
The Frankfurt School
After WW2, a group of Marxists who broke radically from Marxist Orthodoxy formed the Frankfurt School. Books from the group, like Horkheimer’s “Dialectics of Enlightenment” and Adorno’s “Negative Dialectics”, critiqued what they saw as the failure of the Enlightenment project and the problems of modernity. They are generally considered to be the beginning of, or at least precursors to, postmodern thought in continental philosophy.