Kant and "transcendental idealism"
Kant did not call his philosophy 'transcendental idealism.' Transcendental means ‘from without’ or ‘from above’ - an external imposition. Idealism however had a different meaning than the notion of perfection; idealism for Kant and Hegel meant ‘objective and realistic.’ What is not understood today is that the distinction made by Descartes which eliminated the ancient philosophers was the mind-body distinction. Descartes established that the body, or things, or the physical was unknowable and indeterminate. All that could be known was mind - thinking. Descartes also eliminated the soul as “air” by Plato. Descartes distinguished mind as consciousness and eliminated the notion of soul.
Although Descartes lived at the same time as Galileo who was confronted by the Church for proclaiming the blow to man’s egocentricity - man is not the center of the universe, Descartes relied on Aquinas’ cohering of God and science. God could not contradict that a triangle has 3 sides!
Others called Kant's philosophy ‘transcendental idealism’ which Kant frequently rejected. Kant called his philosophy 'critical idealism.' So, when referring to Kant, critical philosophy is invoked, and vice versa, the critical philosophy begins with Kant.
However, concerning transcendental ideas and their use, philosophers often confuse transcendental with metaphysics and logic. Metaphysics and logic are similar because they deal with the science of things grasped in thought; logic being the clarification of ideas as sensible or confused.
Transcendental idealism for Kant is really empirical realism because according to Kant inner reality or thought is impossible without external reality. Kant rejects the idealism of Descartes and Berkeley which holds that all cognition through the senses and experience is sheer illusion.
Kant says, "All cognition of things merely from pure understanding or pure reason is nothing but pure illusion, and only in experience is there truth." Prolegomena to any Future Metaphysics, 1783.
By idealist Kant means 'originated in the subject' and refers directly to the forms of intuition, space and time, and the categories of the understanding which are representations and not things and which organize experience. He considered those who idealized his philosophy by reducing it to logic as retrograde, e.g. Fichte and Schelling.
At issue in Kant's philosophy and in the larger world of philosophy is the problem of the connection between inner and outer, between transcendent ideas which are apriori, that is, Reason, and the immanent or immediate experience. The ground of confusion here is that of illusion and fantasy; this is before the recognition of the social context of thinking and the transmission of thoughts, of language, of a social cognition which deepened the problematic of the connection between thought - in the strict Aristotelian and Kantian sense of categorizing concepts and classifying phenomena - and experience.