Arguments to Idealism

We should note that this is "real" in context of how the meta-phenomenal tradition considered the term (intelligible world, materialism, etc.), where an external environment as seen and felt in experience was demoted to inferior ectype or outright illusion; that is, in favor of the latter having an "invisible" counterpart that was its archetype (an existence "understood" rather than perceived).

Kant actually returned "real" to what was surely the original holder of this status (below), while still also referring at times to "real" (and "objective") in the vein of the other lineage of philosophical thought. Those latter instances leading to confusion of a so-called "noumenal world" being the "true" reality, rather than the connections of the natural world receiving their "being" or verified "be-ing" for the first time in experience. (There's a quote close to the bottom of this post about "The dictum of all genuine idealists from the Eleatic school to..." that clarifies this.) The Moon as it is in itself is arguably "more real" in comparison to its phenomenal presentation, but the empirical world it is represented as a resident of (and thereby made interdependent with other such phenomena) is not surrendering its reality.

Kant ... "The transcendental idealist is, therefore, an empirical realist, and allows to matter, as a phenomenon, a reality which need not be inferred, but may be immediately perceived. The transcendental realism [metaphysical reification of space / time], on the contrary, is necessarily left in doubt, and obliged to give way to empirical idealism [Cartesian skepticism], because it considers the objects of the external senses as something different from the senses themselves, taking mere phenomena as independent beings, existing outside us." ["Outside us" here referring to the second or transcendent meaning of "external"]

"...Thus we see that all external perception proves immediately something real in space, or rather is that real itself. [...] It thus follows, that what is real in external phenomena, is real in perception only, and cannot be given in any other way..." [If intersubjectively witnessed and events conforming to lawfulness or normal expectations.]

"...In our system, therefore, we need not hesitate to admit the existence of matter on the testimony of mere self-consciousness, and to consider it as established by it (i.e. the testimony) [...] External things, therefore, exist by the same right as I myself, both on the immediate testimony of my self-consciousness, with this difference only, that the representation of myself, as a thinking subject, is referred to the internal sense only, while the representations which indicate extended beings are referred to the external sense also. With reference to the reality of external objects, I need as little trust to inference, as with reference to the reality of the object of my internal sense (my thoughts), both being nothing but representations, the immediate perception (consciousness) of which is at the same time a sufficient proof of their reality...."

Disambiguation of the two uses of "external" (empirical world and things in themselves):
"...The transcendental object [thing in itself] is unknown equally in regard to internal and external intuition [inner sense of thoughts and outer sense of objects]. Of this, however, we are not speaking at present, but only of the empirical object, which is called external, if represented in space [extrospective experience], and internal, when represented in temporal relations only [introspective experience], both space and time being to be met with nowhere except in ourselves. The expression, outside us, involves however an inevitable ambiguity, because it may signify either, something which, as a thing by itself, exists apart from us, or what belongs to outward appearance only. In order, therefore, to remove all uncertainty from that concept, taken in the latter meaning (which alone affects the psychological question as to the reality of our external intuition) we shall distinguish empirically external objects from those that may be called so in a transcendental sense, by calling the former simply things occurring in space...."

Mind and matter not separate metaphysical domains, but merely the division of experience as "inner" and "outer", with the supersensible sources for specific objects unknown:
"...No doubt I, as represented by the internal sense in time, and objects in space outside me, are two specifically different phenomena, but they are not therefore conceived as different things. The transcendental object, which forms the foundation of external phenomena, and the other, which forms the foundation of our internal intuition, is therefore neither matter, nor a thinking being by itself, but simply an unknown cause of phenomena which supply to us the empirical concept of both...."

"...But if the psychologist likes to take phenomena for things by themselves, then, whether he admit into his system, as a materialist, matter only, or, as a spiritualist, thinking beings only (according to the form of our own internal sense), or, as a dualist, both, as things existing in themselves, he will always be driven by his mistake to invent theories as to how that which is not a thing by itself, but a phenomenon only, could exist by itself..."

Quotes from Critique of Pure Reason, Friedrich Max Muller translation


Of course, Kant's critical idealism deviated from this. He subsumed minds, monads, the Greeks' intelligible entities, and the materialist's perception-independent version of bodies under a single concept (things in themselves), which -- to whatever extent each could be considered possible -- would have existence independent of our perceptions of them (and accordingly unknowable as to their "is" minus experience). Kant variously addressed those items, and dismissed their authors' contending that they knew the nature of them as things in themselves, via humans having an "intellectual intuition" (and even the treatment of minds or bodies being phenomenal as things in themselves, as in the case of Leibniz's monads and apparently transcendental realism).

Kant ... "Idealism consists in the assertion, that there are none but thinking beings, all other things, which we think are perceived in intuition, being nothing but representations in the thinking beings, to which no object external to them corresponds in fact. Whereas I say, that things as objects of our senses existing outside us are given, but we know nothing of what they may be in themselves, knowing only their appearances, i. e., the representations which they cause in us by affecting our senses. Consequently I grant by all means that there are bodies without us, that is, things which, though quite unknown to us as to what they are in themselves, we yet know by the representations which their influence on our sensibility procures us, and which we call bodies, a term signifying merely the appearance of the thing which is unknown to us, but not therefore less actual. Can this be termed idealism? It is the very contrary."

"...The dictum of all genuine idealists from the Eleatic school to Bishop Berkeley, is contained in this formula: 'All cognition through the senses and experience is nothing but sheer illusion, and only, in the ideas of the pure understanding and reason there is truth.' The principle that throughout dominates and determines my Idealism, is on the contrary: 'All cognition of things merely from pure understanding or pure reason is nothing but sheer illusion, and only in experience is there truth.' But this is directly contrary to idealism proper. How came I then to use this expression for quite an opposite purpose, and how came my reviewer to see it everywhere?..."

"...My so-called (properly critical) Idealism is of quite a special character, in that it subverts the ordinary idealism, and that through it all cognition a priori, even that of geometry, first receives objective reality, which, without my demonstrated ideality of space and time, could not be maintained by the most zealous realists. This being the state of the case, I could have wished, in order to avoid all misunderstanding, to have named this conception of mine otherwise, but to alter it altogether was impossible. It may be permitted me however, in future, as has been above intimated, to term it the formal, or better still, the critical Idealism, to distinguish it from the dogmatic Idealism of Berkeley, and from the skeptical Idealism of Descartes...."

Quotes from Prolegomena To Any Future Metaphysics.

...regardless, both space and time are nonetheless phenomenal objects as it is the very subject which perceives the world and such it goes that there is no object of this world including perception itself that cannot be thought of as a phenomenal event given we can't establish a final chain of command for a source in none of the presented concepts...in all fairness I don't feel anymore clarified in transferring the substance of the world from the object to the subject or to perception itself as none of those can be quite explained for what it really is...thus reducing all concepts to their abstract root all we can assert is a mathematical process of sets and subsets which are functionally related in a system which itself is meaningless, being its solely purpose to constrain or bound the order of these relations...

...so it seams that aside an inclination of mood there is perhaps no big distinction to be made between transcendental idealism and the so called naive realism...
...such distinction can be best qualified as a matter of taste in words, a language game, a territorial dispute, but either way we lean into there is no sign of a "creator of worlds" anywhere to be seen, neither in the subject as an agent, be it God or Man, nor in nature as nature does not bring itself to be if Nature it is...whatever is the case to be true the phenomenal experience is itself an hard fact, which as a set, a thing in itself, although meaningless, it is simultaneously the reason of all meaning !