Kant's philosophy has been called a synthesis of rationalism and empiricism. From rationalism he takes the idea that we can have a priori knowledge of significant truths, but rejects the idea that we can have a priori metaphysical knowledge about the nature of things in themselves, God, or the soul. From empiricism he takes the idea that knowledge is essentially knowledge of experience, but rejects the idea that we cannot learn any necessary truths about experience, and in doing so he rejects Hume's skepticism.
He is able to create this synthesis largely thanks to a radical reconception of the nature of knowledge that comes from from experience. Though empiricists and rationalists may have disagreed about the value or certainty of knowledge from experience, they both generally thought of the mind as a neutral receptor: knowledge from experience was simply the report of the senses. Kant points out that our knowledge of experience extends far beyond what the senses can report. Our senses can report sensations, but they cannot give these sensations a structure in space and time, or organize them according to cause and effect. According to Kant, our faculties or sensibility and understanding are largely responsible for what we think of as "knowledge from experience."
By giving our mind this complex internal structure, Kant makes room for a great deal of a priori knowledge. Though the sensations that are the basis for all our experience come from things in themselves, any regularity or structure we find in these sensations comes from our mental concepts. Thus, while Kant does not slip into the idealist position of saying that reality is all a matter of perception, he does claim that the laws of nature are the laws of our mental faculties. For something to be an objective law, it must be synthetic and it must be a priori, and Kant identifies the possibility of synthetic a priori knowledge within the structure of our mental faculties.
If our sense of order and regularity is not something we find in experience, but something we impose upon experience, the study of this order and regularity is a study of our own faculties rather than a study of experience. Kant reconceives the purpose of metaphysics as being one of critique: we must seek to understand how knowledge is structured, and consequently how the various concepts of our mental faculties are organized. This is an important step for philosophy: after Kant published his work, there was been less interest in making extravagant claims about the nature of the universe, and greater emphasis on determining what we can know and on what grounds we can claim to know it.
This kind of scrutiny, which Kant advocates, has led to some serious attacks on his work. The German Idealists—notably Hegel—were the first to call into question Kant's concept of the thing in itself. Kant insists that while all we can perceive are appearances, these appearances are caused by things in themselves that are outside the realm of experience. Because they are outside the realm of experience, they are also outside the realm of space and time and any of the regularities we perceive in nature. A number of questions can be raised about what kind of relation things in themselves can have with appearances if categories such as time, causation, and even existence do not apply to them. The response of the German Idealists was to abandon the concept of things in themselves and assert that only appearances exist.
Analytic philosophy also got its start by criticizing Kant. This movement criticized in particular his category of the synthetic a priori. Frege was the first to point out that geometry is not synthetic a priori. Pure geometry—which consists only of deductive inferences—is analytic, and empirical geometry—which deals with what space is like in the real world—is knowna posteriori. This position was given a boost by Einstein's relativity, which shows that space is very different from what we had assumed and our understanding of it is certainly not a priori.
Frege also complains that Kant's definition of analytic and synthetic judgments rests on the subject-predicate form of grammar, which is not a necessary part of the logical structure of language. Efforts to define and classify analytic and synthetic judgments have been a major pre-occupation of analytic philosophy, especially in the first half of the twentieth century.
Though many of Kant's doctrines have fallen into question, his exhortation toward critical philosophy remains with us. Perhaps his most lasting contribution is the setting up of a new standard for rigor and circumspection in philosophical investigations.
Kant's Critique of Pure Reason was met mostly with bewilderment when it was first published in 1781. The Prolegomena, published in 1783, was primarily intended to clarify and simplify what was said in the Critique in order to make it accessible. A second, largely revised, edition of the Critique was published in 1787.
Readers and reviewers generally failed to appreciate the originality of Kant's ideas. Readers interpreted Kant as saying something more familiar to them than what he was actually saying. The idea that rationalist metaphysics, which was the main occupation of philosophers in Germany at the time, could be dismissed entirely was too revolutionary a concept to catch on easily.
One of Kant's main troubles, it seems, was that he was taken for an idealist. Idealism is the doctrine that reality is dependent upon the mind. A common idealistic argument suggests that everything I know about the world I learn through the senses, and so the things I "know" are not external objects and phenomena, but just the report of my senses. My concept of the world, an idealist would argue, is based entirely on sensory images that exist only in my mind, and has at best a dubious connection with the things in themselves that exist in the world.
A famous proponent of this position is George Berkeley, an Irish bishop who argues that esse est percipi—"being is being perceived." He asserts that chairs and tables and the like have no independent existence, that they only exist in the mind of someone who is perceiving them. He evades the odd claim that these things cease to exist when no one is perceiving them by positing the existence of God as a being who is perpetually perceiving everything.
Kant's philosophy is very firm in asserting that we can know only about appearances, and that we can know nothing about things in themselves. This assertion is enough to make Kant an idealist of sorts, but he wants to qualify this title of "idealism." He is not, like Berkeley, saying that only appearances exist: though we can know nothing about things in themselves, they are still a crucial part of his philosophy.
Kant calls his philosophy "transcendental" or "critical" idealism. The "transcendent" world of things in themselves is contrasted with the "immanent" world of appearances. Because he believes that things in themselves exist, his idealism believes in the existence of a "transcendent" world that is behind the world of appearances.
His idealism is "critical" because it is directed toward what we can know, not toward what exists. He is not saying that only appearances exist, but that appearances are all we can know. Kant's critical philosophy questions how we can come to know what we know, so he is an idealist only in saying that we cannot know things in themselves.
The concept of the thing in itself is one of the most controversial aspects of Kant's philosophy. In Germany, Kant's successors—most notably Hegel—criticized this concept, and advanced a pure form of idealism that did away with things in themselves entirely. It is unclear what sort of relation things in themselves are supposed to bear to appearances if categories such as space, time, and causality do not apply to them. For instance, suppose we see Frank hit John, and then John hit Frank back, we must assume that these appearances are somehow related to things in themselves. But how can there be actions and reactions in things in themselves without the concept of time?
Because he says that they are unknowable, Kant cannot say anything about the nature of things in themselves, but his silence in this regard leaves us with a number of mystifying puzzles. How can appearances in space and time be related to things in themselves outside of space and time? Those who are dissatisfied with Kant generally identify their dissatisfaction with the unanswered questions raised by Kant's doctrine of things in themselves. Idealists generally deny that things in themselves exist, and realists generally assert that categories like space and time have more than just a subjective existence.
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