The terms “idealism” and “idealist” are by no means used only within philosophy; they are used in many everyday contexts as well. Optimists who believe that, in the long run, good will prevail are often called “idealists”. This is not because such people are thought to be devoted to a philosophical doctrine but because of their outlook on life generally; indeed, they may even be pitied, or perhaps envied, for displaying a naïve worldview and not being philosophically critical at all. Even within philosophy, the terms “idealism” and “idealist” are used in different ways, which often makes their meaning dependent on the context. However, independently of context one can distinguish between a descriptive (or classificatory) use of these terms and a polemical one, although sometimes these different uses occur together. Their descriptive use is best documented by paying attention to the large number of different “idealisms” that appear in philosophical textbooks and encyclopedias, ranging from metaphysical idealism through epistemological and aesthetic to moral or ethical idealism. Within these idealisms one can find further distinctions, such as those between subjective, objective and absolute idealism, and even more obscure characterizations such as speculative idealism and transcendental idealism. It is also remarkable that the term “idealism”, at least within philosophy, is often used in such a way that it gets its meaning through what is taken to be its opposite: as the meaningful use of the term “outside” depends on a contrast with something considered to be inside, so the meaning of the term “idealism” is often fixed by what is taken to be its opposite. Thus, an idealist is someone who is not a realist, not a materialist, not a dogmatist, not an empiricist, and so on. Given the fact that many also want to distinguish between realism, materialism, dogmatism, and empiricism, it is obvious that thinking of the meaning of “idealism” as determined by what it is meant to be opposed to leads to further complexity and gives rise to the impression that underlying such characterizations lies some polemical intent.Within modern philosophy there are sometimes taken to be two fundamental conceptions of idealism: 1. something mental (the mind, spirit, reason, will) is the ultimate foundation of all reality, or even exhaustive of reality, and 2. although the existence of something independent of the mind is conceded, everything that we can _know_ about this mind-independent “reality” is held to be so permeated by the creative, formative, or constructive activities of the mind (of some kind or other) that all claims to knowledge must be considered, in some sense, to be a form of self-knowledge.Idealism in sense (1) has been called “metaphysical” or “ontological idealism”, while idealism in sense (2) has been called “formal” or “epistemological idealism”. The modern paradigm of idealism in sense (1) might be considered to be George Berkeley’s “immaterialism”, according to which all that exists are ideas and the minds, less than divine or divine, that have them. (Berkeley himself did not use the term “idealism”.) The fountainhead for idealism in sense (2) might be the position that Immanuel Kant asserted (if not clearly in the first edition of his _Critique of Pure Reason_ (1781) then in his _Prolegomena to Any Future Metaphysics_ (1783) and in the “Refutation of Idealism” in the second edition of the _Critique_) according to which idealism does “not concern the existence of things”, but asserts only that our “modes of representation” of them, above all space and time, are not “determinations that belong to things in themselves” but features of our own minds. Kant called his position “transcendental” and “critical” idealism, and it has also been called “formal” idealism. However, Kant’s position does not provide a clear model of idealism at all. While Kant himself claimed that his position combined “empirical realism” with “transcendental idealism”, that is, combined realism about external, spatio-temporal objects in ordinary life and science with the denial of the reality of space and time at the level of things as they are in themselves, it also insisted upon the reality of things as they are in themselves existing independently from our representations of them, thus denying their reducibility to representations or the minds that have them. In this way, Kant’s position actually combines the transcendental ideality of space and time with a kind of realism about the existence of things other than minds. So instead of using Kant as any kind of model for epistemological idealism, in this entry we will distinguish between metaphysical and epistemological arguments for idealism understood as a metaphysical doctrine, namely that everything that exists is in some way mental. We thus agree with A.C. Ewing, who wrote in 1934 that all forms of idealism > have in common the view that there can be no physical objects existing apart from some experience, and this might perhaps be taken as the definition of idealism, provided that we regard thinking as part of experience and do not imply by “experience” passivity, and provided we include under experience not only human experience but the so-called “Absolute Experience” or the experience of a God such as Berkeley postulates. (Ewing 1934: 3) in other words, while reducing all reality to some kind of perception is one form of idealism, it is not the only form—reality may be reduced to the mental on other conceptions of the latter. Thus Willem deVries’s more recent definition of idealism as the general theory that reduces reality to some form or other of the mental is just: > Roughly, the genus comprises theories that attribute ontological priority to the mental, especially the conceptual or ideational, over the non-mental. (deVries 2009: 211)