The world, or our representations?

Berkeley takes scepticism to be the disease to cure, and immaterialism to be its remedy. Modern doubt, he says, springs entirely from matter: once one posits real things hidden behind our ideas, one can never know whether our ideas resemble them, hence the veil of perception into which the sceptic rushes. Remove that unknowable matter, and the gap vanishes: things are no more than what we perceive, immediately and with certainty. Far from the rationalist dogmatism of Descartes, who grounds certainty on reason, Berkeley relocates it in sensible perception itself, made secure because there is no longer anything outside it to miss.

Three Dialogues between Hylas and Philonous, Preface and III · Principles of Human Knowledge, §§86-88

Berkeley starts from a simple empiricist remark: whatever I have an idea of, I have it through the senses, hence as a perception. But to posit, behind my perceptions, a matter that would exist without being perceived is to posit the idea of a thing of which, by definition, I can have no idea: a contradiction. He thus turns Locke's empiricism against matter itself: what we call bodies is only an ordered collection of sensible ideas, and for them to exist is to be perceived. What remains is that which perceives, the mind, the one genuine substance. Far from denying the world, he claims to save it from scepticism by removing the veil of an unknowable matter set between us and it.

Their being is to be perceived; nor is it possible they should have any existence out of the minds which perceive them.Principles of Human Knowledge, §3

Principles of Human Knowledge, §§3-7 · Three Dialogues between Hylas and Philonous, I

Roused from his dogmatic slumber by Hume, Kant rejects both opposing paths: dogmatic metaphysics, which claims to know the unconditioned, God, the soul, the world as a totality, loses itself in the antinomies, where reason contradicts itself; yet Humean skepticism, dissolving necessity into habit, is just as untenable. The critical path settles it: there is indeed an apodictic certainty, that of synthetic a priori judgments, but it holds only of appearances, objects as they appear to us under the forms of the subject, not of things in themselves. A necessary and universal certainty, then, but paid for with an impassable limit.

I freely admit that it was the objection of David Hume that, many years ago, first interrupted my dogmatic slumber.Prolegomena, Preface

Critique of Pure Reason, Prefaces and Transcendental Dialectic · Prolegomena to Any Future Metaphysics

If the mind conformed to objects, we could never know anything of them a priori; reverse the hypothesis, as Copernicus set the observer at the centre, and let objects conform to our cognition. Then space, time and the categories are not features of things but the forms of the subject under which alone an object can appear to us. The consequence is transcendental idealism: we know only phenomena, never the thing in itself. This boundary saves science (its laws hold necessarily for every phenomenon) without collapsing into Berkeley's idealism, since the thing in itself remains; the phenomenon / noumenon dualism thus kept rules out the fully idealist pole.

Critique of Pure Reason, Transcendental Aesthetic · Prolegomena to Any Future Metaphysics, §13