Transcendental idealism

Transcendental idealism is a doctrine founded by the 18th-century German philosopher] Immanuel Kant. Kant presents it as the point of view which holds that our experience of things is about how they appear to us, not about those things as they are in and of themselves.

Background
Despite this influence, it was a subject of some debate amongst 20th century philosophers exactly how to interpret this doctrine, which Kant first describes in his Critique of Pure Reason. Kant distinguished his view from contemporary views of realism and idealism, but philosophers are not agreed upon what difference Kant draws.

Transcendental idealism is occasionally identified with formalistic idealism on the basis of passages from Kant's Prolegomena to any Future Metaphysics, although recent research has tended to dispute this identification. Transcendental idealism was also adopted as a label by Fichte and Schelling and reclaimed in the 20th century in a different manner by Husserl.

Transcendental idealism denies that we could have knowledge of the thing in itself. A view that holds the opposite is called transcendental realism.

Note that Xenophanes of Colophon in 530 BCE came up with something that could be considered an ancestor to Kant's epistemology: "And as for certain truth, no man has seen it, nor will there ever be a man who knows about the gods and about all the things I mention. For if he succeeds to the full in saying what is completely true, he himself is nevertheless unaware of it; and Opinion (seeming) is fixed by fate upon all things." (From Kathleen Freeman's Ancilla to the Presocratic Philosophers, Xenophanes fragment 34.)

Some interpretations of some of the medieval Buddhists of India, such as Dharmakirti, may reveal them to be transcendental idealists, since they seemed to hold the position of mereological nihilism but where minds are distinct from the atoms. Some Buddhists often attempt to maintain that the minds are equal to the atoms of mereological nihilist reality, but Buddhists seem to have no explanation of how this is the case, and much of the literature on the aforementioned Buddhists involves straightforward discussion of atoms and minds as if they are separate. This makes their position very similar to transcendental idealism, resembling Kant's philosophy where there are only things-in-themselves (which are very much like philosophical atoms), and phenomenal properties.

Schopenhauer
Some of Schopenhauer's comments on the definition of the word "transcendental" are as follows:

Schopenhauer contrasted Kant's transcendental critical philosophy with Leibniz's dogmatic philosophy.

P. F. Strawson
In The Bounds of Sense, P. F. Strawson suggests a reading of Kant's first Critique which rejects most of its arguments, including transcendental idealism. Strawson views the analytic argument of the transcendental deduction as the most valuable idea in the text, determining transcendental idealism to be a great but unavoidable error in Kant's system. In this traditional reading (also favored in the work of Paul Guyer and Rae Langton), the Kantian term phenomena (literally something that can be seen from the Greek word phainomenon, "observable") refers to the world of appearances, or the sensible. The necessary preconditions of experience, such as space and time, are what make a priori judgements possible, but all of this only applies to human sensibility. Kant's system requires the existence of noumena to prevent a rejection of external reality altogether, and it is this concept (senseless objects of which we can have no real understanding) to which Strawson objects in his book.

Henry Allison
In Kant's Transcendental Idealism, Henry Allison proposes a reading in opposition to Strawson's interpretation. Allison argues that Strawson and others take Kant too literally in discussing a world of phenomena, making the doctrine of transcendental idealism seem untenable by assuming that Kant vacillated between two different concepts of "appearance." In Allison's reading, Kant's view is better characterized as a two-aspect theory, where noumena and phenomena refer to aspects of a single reality, and thus Kant is a neutral monist. It is the discursive character of human sensibility rather than epistemological humility that Kant asserted.