Possible worlds
One kind of modal realism holds that there is a distinctive class of truths essentially involving the modal notions of necessity and possibility. Since the mid-20th century, however, advances in modal logic—in particular the development of possible-world semantics—have given rise to a further, distinctively ontological dispute concerning whether that semantics gives a literally correct account of the “truth-conditions” of modal propositions. According to possible-world semantics, (1) a proposition is necessarily true if (and only if) it is true not only in the actual world but in all possible worlds; and (2) a proposition is possibly true if and only if it is true in at least one possible world, perhaps distinct from the actual world. If statements 1 and 2 are literally correct descriptions of the truth-conditions of modal propositions, then, if any truths are nontrivially necessary or correctly assert unrealized possibilities, there must exist, in addition to the actual world, many other merely possible worlds. Modal realism, in the uncompromising form defended by the American philosopher David Lewis, is the view that there exists a (very large) plurality of worlds, each of which is a spatiotemporally (and therefore causally) closed system, disjoint from all others and comprising its own distinctive collection of concrete particulars, replete with all their properties and relations to each other.
Although Lewis’s worlds are not, as he conceived them, abstract entities, it is clear that his realism faces epistemological objections similar to those mentioned in connection with abstracta. These, along with other considerations, led some philosophers to propose alternatives designed to secure the benefits of possible-world semantics without the costs of full-blooded realism. The alternatives included a more moderate realism propounded by the American philosopher Robert Stalnaker which denies Lewis’s homogeneity thesis (the claim that merely possible worlds are entities of the same kind as the actual world), as well as fictionalism, the view that possible-world theory is literally false but useful.
Realism and idealism
The opposition between idealism and realism, although undeniably ontological in a broad sense, is distinct both from general disputes about realism in ontology and from disputes which turn upon the notion of truth or its applicability to statements of some specified type (see below Realism and truth). In its most straightforward and, arguably, basic sense, idealism not only asserts the existence of “ideas” (and perhaps other mental entities) but also advances a restrictive claim about the nature or composition of reality as whole: there is nothing in reality other than ideas and the minds whose ideas they are. So understood, idealism is a form of monism, which is opposed both to other forms of monism (e.g., materialism) and to pluralism, which posits two or more irreducibly distinct kinds of stuff or things (e.g., mental and physical, as in various versions of dualism).
A paradigmatic example of an idealist position is Berkeley’s rejection of “brute matter” as unintelligible and his accompanying doctrine that reality consists exclusively of “ideas”—for which esse est percipi (“to be is to be perceived”)—and “spirits,” including finite spirits corresponding to individual human beings and at least one infinite spirit, or God. If idealism in this sense is to be viewed as a kind of antirealism, the realism it opposes must be one which maintains the existence of material things independently of their being perceived or otherwise related to any mind, finite or otherwise.
The 18th-century German philosopher Immanuel Kant recognized that Berkeley’s “dogmatic idealism” involved denying the independent reality of space. Berkeley’s arguments, he thought, were effective against metaphysical positions which assumed that space is a property of “things in themselves,” as opposed to their representations, or “appearances,” in the mind. Kant argued to the contrary that space as well as time are forms of “sensible intuition,” or the mode in which the mind is affected by sensible objects. Thus, the reality of objects external to the mind (objects in space) is guaranteed, because being in space and time is a condition of being an object of sensible experience at all. Kant’s combination of transcendental idealism—the doctrine that what is given in experience are only appearances—with empirical realism—the view that there are objects external to the mind—allowed him to reject the conception of external objects as “lying behind” appearances and as knowable only (if at all) by a problematic and ultimately indefensible inference from what is given in experience to its hidden causes.
The views of G.E. Moore (1873–1958) were appreciably closer to commonsense realism about the external world than were Kant’s. Although reacting, especially in his early papers, primarily against the prevailing tradition of 19th-century British idealism, Moore criticized Berkeley’s esse est percipi doctrine while at the same time rejecting Kant’s transcendental idealism.