It is a mark of courage or at least bold if not heroic confidence that Kant forged ahead with his attempt to describe the mechanisms and functions of human cognition, even though he suspected that some of the descriptive ideas he invented would probably never be scientifically verified.
In a section of the Critique of Pure Reason, titled “The Schematism of the Pure Concepts of the Understanding” (chapter 1 of book 2 of the “Transcendental Analytic”) he wrote:
This schematism of our understanding, in its application to appearances and their mere form, is an art concealed in the depths of the human soul, whose real modes of activity nature is hardly likely ever to allow us to discover, and to have open to our gaze.
Preceding this pessimistic observation, Kant had been explaining his hypothesized “schema” as rules for how to conjoin or link an a priori concept that is not dependent on sensual experience with an object of perception obtained through the senses. He uses several examples to illustrate. With the example of a triangle he asserts that associating the concept of triangle with an empirically apprehend image of a triangle is effected by means of set of innate rules or instructions that he calls a schema. A “schematism” according to Kant is an application of a set of rules whereby a pure, a priori concept of understanding such as space, time, triangle, or number is correlated with an empirically constituted object or idea.
But in the interjected “aside” to the reader quoted above, Kant expresses his suspicion that “nature is hardly likely ever to allow us to discover” the “real modes of activity” that constitute the functioning of those schema, or rules for linking the inner, mental world of pure concepts with sensibly perceived objects or empirically derived ideas.
Kant may have been trying with his schema — sets of rules for linking a priori concepts with sensually perceived objects — to (among other things) heal the body/mind split of Descartes. If Kant was right about “nature” being likely to never allow us to “gaze” upon schema, then neuroscientists who today address the hard problem of consciousness by mapping the areas of the brain that correlate with subjectively reported states of consciousness (qualia), observed behavior, or visible correlates of autonomic nerve activity, these neuroscientists may then, like Kant, worry that they too will likely never succeed in actually “gazing” on the mechanisms (schema, rules) that link inner consciousness experience with the exterior world of things.
More than 230 years following the publication of Kant’s Critique of Pure Reason, many philosophers and cognitive scientists working with or following the ideas of philosophers like Daniel Dennett, prefer not to invent and use terms or ideas like Kant’s “schema” — the hypothesized (invisible, postulated, unverifiable) rules for how we link or associate inner concepts with perceived objects. They avoid and argue against using scientifically unverifiable ideas or hypotheses. They, like Dennett, will for the sake of argument deign to take up for discussion ideas that are not susceptible to empirical proof (cf. the ether and phlogiston hypotheses), but then, after discussing and exposing their vacuousness and unusability, summarily drop them, and urge others to do the same.
Knowing what terms or ideas to avoid using in attempting scientifically to solve the riddle and mystery of consciousness is important. Knowing what not to do or think is progress. But the hard problem of consciousness (what is like to be something; qualia; John Searle’s Chinese room; Mary the color-blind super scientist) is still unsolved. Notwithstanding his occasional use of non-verifiable ideas like “schema” and “schematism,” Kant’s attempts in the Critique to heal the mind-body split deduced by René Descarte in his Meditations, argue against the empirical skepticism expressed by David Hume in his Treatise and Enquiry, and correct or balance the radical empiricism of John Locke’s Essay surely contain many useable ideas that can be “operationalized” by neuroscientists, cognitive researchers, and philosophers of consciousness in their attempt to solve, understand, and explain the “hard problem” of consciousness.
A book published many years ago by Robert Paul Wolff, Kant’s Theory of Mental Activity: A Commentary on the Transcendental Analytic of the Critique of Pure Reason (Harvard University Press, 1963; Kindle [Society for Philosophy and Culture, 2013]) is the best commentary I have yet read on Kant’s Critique. Wolff has also recorded and published on YouTube a very helpful nine-part lecture series on the Critique.