Beyond this, idealists disagree on which aspects of the mental are more basic.
– Platonic idealism affirms that ABSTRACTIONS are more basic to reality than the things we perceive, / while subjective idealists and phenomenalists tend to privilege SENSORY EXPERIENCE over ABSTRACT REASONING.
– Subjective idealists like George Berkeley are anti-realists in terms of a mind-independent world, / whereas transcendental idealists like Immanuel Kant are strong skeptics of such a world, affirming epistemological and not metaphysical idealism.
– Thus Kant defines idealism as “the assertion that we can never be certain whether all of our putative outer experience is not mere imagining”.[Immanuel Kant, Notes and Fragments, ed. Paul Guyer, trans. by Curtis Bowman, Paul Guyer, and Frederick Rauscher, Cambridge University Press, 2005, p. 318, ISBN 0-521-55248-6]
– He claimed that, according to idealism, “the reality of external objects does not admit of strict proof. On the contrary, however, the reality of the object of our internal sense (of myself and state) is clear immediately through consciousness.” [Critique of Pure Reason, A 38]
However, not all idealists restrict the real or the knowable to our immediate subjective experience.
Objective idealists make claims about a transempirical world, but simply deny that this world is essentially divorced from or ontologically prior to the mental. Thus Plato and Gottfried Leibniz affirm an objective and knowable reality transcending our subjective awareness—a rejection of epistemological idealism—but propose that this reality is grounded in ideal entities, a form of metaphysical idealism. Nor do all metaphysical idealists agree on the nature of the ideal; for Plato, the fundamental entities were non-mental abstract forms, while for Leibniz they were proto-mental and concrete monads.[Mark Kulstad and Laurence Carlin, “Leibniz’s Philosophy of Mind”, Stanford Encyclopedia of Philosophy, http://plato.stanford.edu/entries/leibniz-mind]
As a rule, transcendental idealists like Kant affirm idealism’s epistemic side without committing themselves to whether reality is ultimately mental; objective idealists like Plato affirm reality’s metaphysical basis in the mental or abstract without restricting their epistemology to ordinary experience; and subjective idealists like Berkeley affirm both metaphysical and epistemological idealism.[http://www.enotes.com/science-religion-encyclopedia/idealism]