Between Spinoza and Kant
Among the various philosophers who influenced Marx, Nietzsche, and Freud, I want to single out, for now, Kant and Spinoza. Kant and Spinoza stand out because they adopted radically different solutions to a common problem, and because both solutions were attractive to Marx, Nietzsche, and Freud.
The problem concerns how to think about the place of humans in the natural world, a problem that took on new form and importance in the intellectual and cultural upheavals of the 17th century (the scientific revolution, the fact of religious pluralism, and the rise of bourgeois capitalism).
The Spinozistic solution is to think of humans as nothing more than a piece of the natural world. At the beginning of Book III of the Ethics, Spinoza denounces those who want to think of humanity as "an empire within an empire" -- as something distinct. Spinoza then goes on to use his theory of physics (how bodies affect other bodies) to develop a theory of the emotions, virtues, and vices.
The Kantian solution is to think of humans as not merely outside of the natural world but also as (somehow) creating it. Space and time are "forms of intuition"; cause and substance are "categories of the understanding." The world as it is in itself -- including "the real us" -- is non-temporal, non-spatial, non-causal, and non-substantial -- and so entirely unknowable and yet thinkable (indeed, necessarily thinkable!).
Thus, while Kant might concede to Spinoza that, with respect to knowledge, humans are only a bit of nature, nevertheless we also stand outside of the natural (=knowable) world. Kant shows how our capacity to stand outside of the knowable (=natural) world is deeply connected with our "spontaneity," i.e. our capacity to judge, to evaluate, and to think.
So, on the one hand, a picture of human beings as nothing other than bits of nature and entirely knowable in those terms (Spinoza); on the other, a picture of human beings as standing outside of nature in some way that we can think but cannot know (Kant).
To borrow terms from John McDowell's Mind and World (1991), there is here a forced choice between "bald naturalism" (Democritus, Spinoza, Quine) and "rampant platonism" (Plato, Kant, Husserl). Yet McDowell wants to discern a neglected alternative, "naturalized platonism," and to this end enlists Aristotle and Hegel.I think that Marx, Nietzsche, and Freud might also be thought of as "naturalized platonists," in the following sense: on the one hand, each wants to recognize the distinctiveness of human cultural achievements; on the other hand, each wants to stress that these achievements are made possible due to conditions which differ in degree, but not in kind, from conditions that hold elsewhere in the natural world.