Transcendental idealism
Transcendental idealism is a doctrine founded by German philosopher Immanuel Kant in the 18th century. Kant's doctrine is found throughout his Critique of Pure Reason. Kant argues that the conscious subject cognizes objects not as they are in themselves, but only the way they appear to us under the conditions of our sensibility. Thus Kant's doctrine restricts the scope of our cognition to appearances given to our sensibility and denies that we can possess cognition of things as they are in themselves, i.e. things as they are independently of how we experience them through our cognitive faculties.
Space, time and causality—the necessary ways in which phenomena are related to one another—do not have an existence 'outside' of us, separate from phenomena. Rather, like the phenomena, these forms of interrelatedness are mind-dependent, i.e. originate in our mental faculties. Kant's doctrine is commonly presented as the notion that time, space, and causality are not independently existing entities, but constitute the necessary mental preconditions of experiencing the world.
Background
Although it influenced the course of subsequent German philosophy dramatically, exactly how to interpret this concept was a subject of some debate among 20th century philosophers. Kant first describes it in his Critique of Pure Reason, and distinguished his view from contemporary views of realism and idealism, but philosophers do not agree how sharply Kant differs from each of these positions.Transcendental idealism is associated 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 the subsequent German philosophers Johann Gottlieb Fichte and Friedrich Wilhelm Joseph von Schelling, Arthur Schopenhauer, and in the early 20th century by Edmund Husserl in the novel form of transcendental-phenomenological idealism.
Kant's transcendental idealism
Kant presents an account of how we intuit objects and accounts of space and of time. Before Kant, some thinkers, such as Leibniz, had come to the conclusion that space and time were not things, but only the relations among things. Contrary to thinkers, including Newton, who maintained that space and time were real things or substances, Leibniz had arrived at a radically different understanding of the universe and the things found in it. According to his Monadology, all things that humans ordinarily understand as interactions between and relations among individuals have their being in the mind of God but not in the Universe where we perceive them to be. In the view of realists, individual things interact by physical connection and the relations among things are mediated by physical processes that connect them to human brains and give humans a determinate chain of action to them and correct knowledge of them.Kant was aware of problems with the positions of both of these thinkers. He had been influenced by the physics of Newton and understood that there is a physical chain of interactions between things perceived and the one who perceives them. However, an important function of mind is to structure incoming data and to process it in ways that make it other than a simple mapping of outside data.
The salient element here is that space and time, rather than being real things-in-themselves or empirically mediated appearances, are the very forms of intuition by which we must perceive objects. They are hence neither to be considered properties that we may attribute to objects in perceiving them, nor substantial entities of themselves. They are in that sense subjective, yet necessary, preconditions of any given object insofar as this object is an appearance and not a thing-in-itself. Humans necessarily perceive objects as located in space and in time. This condition of experience is part of what it means for a human to cognize an object, to perceive and understand it as something both spatial and temporal: "By transcendental idealism I mean the doctrine that appearances are to be regarded as being, one and all, representations only, not things in themselves, and that time and space are therefore only sensible forms of our intuition…" Kant argues for these several claims in the section of the Critique of Pure Reason entitled the "Transcendental Aesthetic". That section is devoted to inquiry into the a priori conditions of human sensibility, i.e. the faculty by which humans intuit objects. The following section, the "Transcendental Logic", concerns itself with the manner in which objects are thought.
Historical parallels
of Colophon in 530 BC anticipated Kant's epistemology in his reflections on certainty. "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 is fixed by fate upon all things."Certain 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 transcendental idealists who held that their minds were 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, and phenomenal properties.
Schopenhauer
Schopenhauer takes Kant's transcendental idealism as the starting point for his own philosophy, which he presents in The World as Will and Representation. Schopenhauer described transcendental idealism briefly as a "distinction between the phenomenon and the thing in itself", and a recognition that only the phenomenon is accessible to us because "we know neither ourselves nor things as they are in themselves, but merely as they appear." Some of Schopenhauer's comments on the definition of the word "transcendental" are as follows:Further on in §13, Schopenhauer says of Kant's doctrine of the ideality of space and time: "Before Kant, it may be said, we were in time; now time is in us. In the first case, time is real and, like everything lying in time, we are consumed by it. In the second case, time is ideal; it lies within us."
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 that, once accepted, forces rejection of most of the original arguments, including transcendental idealism. Strawson contends that, had Kant followed out the implications of all that he said, he would have seen that there were many self-contradictions implicit in the whole.Strawson views the analytic argument of the transcendental deduction as the most valuable idea in the text, and regards transcendental idealism as an unavoidable error in Kant's greatly productive system. In Strawson's traditional reading, the Kantian term phenomena refers to the world of appearances, or the world of "things" sensed. They are tagged as "phenomena" to remind the reader that humans confuse these derivative appearances with whatever may be the forever unavailable "things in themselves" behind our perceptions. The necessary preconditions of experience, the components that humans bring to their apprehending of the world, the forms of perception such as space and time, are what make a priori judgments possible, but all of this process of comprehending what lies fundamental to human experience fails to bring anyone beyond the inherent limits of human sensibility. Kant's system requires the existence of noumena to prevent a rejection of external reality altogether, and it is this concept to which Strawson objects in his book.