Transcendental idealism is the name given by eighteenth century German philosopher Immanuel Kant to the epistemological approach of his philosophy. Kant presents it as the point of view which holds that one's experience of things is about how they appear to that person, not about those things as they are in and of themselves.
Transcendental idealism represents Kant’s attempt at a synthesis between existing forms of idealism (affirming the reality of a spiritual or ideal realm above that of material reality) and empiricism (affirming the precedence of sense perception over idealistic of spiritual speculation). Kant’s transcendental method bases its approach on the acknowledgment of a priori (transcendental) mental functions that determine the way people process any information. This method both stresses the reliability of science and the inability to know what transcends observable phenomena. In a sense, it is thus a form of skepticism. The main challenge to Kant’s approach has been that it does not clearly show how it is possible to both affirm the existence of an independent reality and state that nothing can be said about it.
Perhaps the best way to approach transcendental idealism is by looking at Kant's account of how people intuit (anschauen) objects. What is relevant here is that space and time, rather than being real "things-in-themselves" or empirically mediated appearances (Erscheinungen), are the very "forms of intuition" (Anschauung), i.e., the way people perceive objects. Time and space are hence neither to be considered properties that observers may attribute to objects in perceiving them, nor substantial entities of themselves. In that sense, they are subjective, yet necessary preconditions of any given object insofar as that object is an appearance and not a "thing-in-itself." Humans necessarily perceive objects spatially and temporally. This is part of what it means for a human to cognize an object, to perceive it as something both spatial and temporal. These are all claims Kant argues for in the section of the Critique of Pure Reason entitled the "Transcendental Aesthetic." This section is devoted to the inquiry of the a priori conditions of (human) sensibility, i.e. the faculty by which objects are apprehended. The following section, the "Transcendental Logic" concerns itself with the manner in which objects are thought through the so-called a priori categories of understanding (such as the notion of causality).
With regard to the adjective "transcendental," Kant defined it in the following way when he used it to describe knowledge:
I call all knowledge transcendental if it is occupied, not with objects, but with the way that we can possibly know objects, even before we experience them. (Critique of Pure Reason, A12, B26)
Kant himself offers a definition of his transcendental idealism and asks, rhetorically, how it is different from what is traditionally known as idealism.
As the senses … never and in no single instance enable us to know things in themselves, but only their appearances, and as these are mere representations … all bodies, together with the space in which they are, must be held to be nothing but mere representations in us, and exist nowhere else than merely in our thought. Now is this not manifest idealism? (Prolegomena to any Future Metaphysics § 13, Note II)
Kant answered this question in the negative. His grounds were that he did not deny that there are things-in-themselves but only that people can know anything of them as they are. In other words, reality, as it is known, is determined by forms of intuition (space and time) and the categories of reason, but there is something "out there:" Kant never accepted the conclusion of what he called idealism and is generally known for his subjective idealism (proposed by George Berkeley), i.e. that reality, being known only through the mind, also exists only through the mind, which naturally tends towards a form of solipsism. If reality of external things, independently from the perception of them, is denied, only the “I” of the philosopher remains real.
A few years later, the German idealists would pursue a similar direction, but on entirely different premises.
The transcendental idealist, says Kant, can afford to be a realist on the empirical level. When saying that external things are “real,” he does nothing more than say that they are real within the necessary conditions of the human faculties of thought and intuition.
[E]verything intuited or perceived in space and time, and therefore all objects of a possible experience, are nothing but phenomenal appearances, that is, mere representations, which in the way in which they are represented to us, as extended beings, or as series of changes, have no independent, self-subsistent existence apart from our thoughts. This doctrine I entitle transcendental idealism. (Critique of Pure Reason, A491, B520)
Empirical science, Kant continues, can be trusted (to the extent that it is properly conducted), because it merely recognizes that the laws of the mind apply to the sensory perceptions by the forms of intuition (time and space) of the mind. Science make no claim about what things ultimately are, it does not deal with their metaphysical significance, and most especially it makes no claims about notions that do not correspond to any sensory perception (God, eternal life).
Transcendental idealism, Kant’s own philosophical stance, thus makes a preemptive strike against all illusory assumptions: Anything that is known about things is only what is known through the vision of the mind’s laws. Within this caveat, once taken into consideration, the philosopher and scientist is free to apply these laws for practical purposes. He remains agnostic about their ultimate or absolute meaning.
On the other hand, Kant distinguishes his position from dogmatic or skeptical philosophy by invoking the distinction between transcendental idealism and transcendental realism. Kant succinctly defined transcendental idealism in this way:
A transcendental realist mistakenly considers space, time, and objects alike, to be real in themselves, quite independently from the human perception of them. This is the case for dogmatism (Leibniz) and empiricism (Locke) alike. Both must, according to Kant, consider appearances—the spatial-temporal objects of everyday experience—as imperfect shadows of a transcendent reality. Indeed, if one considers that objects exist in space and time in themselves, one is always left to wonder whether his or her ideas really correspond to the objects. The dogmatist will be forced to make arbitrary decisions and the empiricist will end up in skepticism: Like Hume, he will come to doubt every rational inference of the mind.
The conclusion is obvious for Kant: His transcendental idealism is superior in every way. Precisely because it does not make claims it cannot sustain about the ultimate reality of things (including time and space), it leaves one free to make definite statements about things to the extent they appear to the observer through the forms of intuition and the categories of the mind.
In his Critique of Pure Reason (A482, B520) and in his Prolegomena to any Future Metaphysics, Kant indicates that it might be more appropriate to use the terms “formal(istic) idealism” or “critical idealism” to avoid confusion with the usual idealism that doubts the existence of outer things.
Transcendental idealism was also adopted as a label by Fichte and Schelling; it was maintained as a key notion by the various Kantian and neo-Kantian schools and reclaimed in the twentieth century in a different manner by Husserl. For all their differences, these philosophies all claim the primacy of the human mind’s activity over external sensory perception in the process of cognition.
Though, in the end, he submitted some of Kant’s views to a severe critique, nineteenth century German philosopher Arthur Schopenhauer fully endorsed the approach of transcendental idealism. Since Schopenhauer is rightly known for the clarity of his presentations, it is worth quoting his comments on the definition of the word "transcendental:"
Transcendental is the philosophy that makes us aware of the fact that the first and essential laws of this world that are presented to us are rooted in our brain and are therefore known a priori. It is called transcendental because it goes beyond the whole given phantasmagoria to the origin thereof. Therefore, as I have said, only the Critique of Pure Reason and generally the critical (that is to say, Kantian) philosophy are transcendental. (Parerga and Paralipomena, Volume I, "Fragments for the History of Philosophy," § 13)
Schopenhauer contrasted Kant's transcendental critical philosophy with Leibniz's dogmatic philosophy.
With Kant the critical philosophy appeared as the opponent of this entire method [of dogmatic philosophy]. It makes its problem just those eternal truths (principle of contradiction, principle of sufficient reason) that serve as the foundation of every such dogmatic structure, investigates their origin, and then finds this to be in man's head. Here they spring from the forms properly belonging to it, which it carries in itself for the purpose of perceiving and apprehending the objective world. Thus here in the brain is the quarry furnishing the material for that proud, dogmatic structure. Now because the critical philosophy, in order to reach this result, had to go beyond the eternal truths, on which all the previous dogmatism was based, so as to make these truths themselves the subject of investigation, it became transcendental philosophy. From this it follows also that the objective world as we know it does not belong to the true being of things-in-themselves, but is its mere phenomenon, conditioned by those very forms that lie a priori in the human intellect (i.e., the brain); hence the world cannot contain anything but phenomena. (The World as Will and Representation, Vol. I, Appendix: "Criticism of the Kantian Philosophy")
The groundbreaking character of Kant’s transcendental idealism has hardly been denied by anyone. Nevertheless, it is far from having been universally accepted as a satisfactory solution to the problems of epistemology.
The crux of Kant’s entire theoretical philosophy is that it affirms that one cannot know anything about “things-in-themselves” and at the same time affirms that things in themselves must certainly exist. Kant was particularly opposed to George Berkeley’s subjective idealism, because that form of idealism denied the existence of things apart from the subject (divine or human) perceiving them. Kant was very much intent on stressing the difference between these views and his own philosophy to avoid being considered a dreamer (which Berkeley was in his mind). However, Kant’s often unfair assessment of Berkeley might be due to his awareness that his own philosophy had a weakness that might easily lead one to believe that he was in agreement with subjective idealism.
How it is possible to affirm the existence of something which one also affirms being unable to say anything about is a problem that has been discussed abundantly by successive generations of thinkers. For Kant, the awareness of things around one comes directly together with one's self-awareness, thus the existence of the external world was as certain as the existence of the “I.” It is simply impossible to know what things are in themselves.
In The Bounds of Sense, P. F. Strawson argues that the things-in-themselves or noumenon are the building blocks upon which Kant’s entire system rests, and that their very notion is not acceptably justified by Kant. Henry Allison's reading, on the other hand, is that Kant's view is better characterized as a two-aspect theory, where noumena and phenomena refer to different ways of considering an object.
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