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In metaphysics, a universal is what particular things have in common, namely characteristics or qualities. In other words, universals are generalizations whose truths tend to apply on net, though particular exceptions may exist, that are repeatable or recurrent entities that can be instantiated or exemplified by many particular things.〔Price (1953); Loux (1998), p 20.〕 For example, suppose there are two chairs in a room, each of which is green. These two chairs both share the quality of "being a chair," as well as greenness or the quality of being green. Metaphysicians call this quality that they share a "universal." There are three major kinds of qualities or characteristics: types or kinds (e.g. mammal), properties (e.g. short, strong), and relations (e.g. father of, next to). These are all different types of universal.〔Loux (2001), p. 4.〕 Paradigmatically, universals are ''abstract'' (e.g. humanity), whereas particulars are ''concrete'' (e.g. the person of Socrates). However, universals are not necessarily abstract and particulars are not necessarily concrete.〔Rodriguez-Pereyra (2008), §1.〕 For example, one might hold that numbers are particular yet abstract objects. Likewise, some philosophers, such as D.M. Armstrong, consider universals to be concrete. Most do not consider classes to be universals, although some prominent philosophers do, such as John Bigelow. ==Problem of universals== (詳細はproblem of universals'' is an ancient problem in metaphysics about whether universals exist. The problem arises from attempts to account for the phenomenon of similarity or attribute agreement among things.〔Loux (1998), p. 20; (2001), p. 3.〕 For example, live grass and Granny Smith apples are similar or agree in attribute, namely in having the attribute of greenness. The issue is how to account for this sort of agreement in attribute among things. There are many philosophical positions regarding universals. Taking "beauty" as an example, three positions are: * Idealism or conceptualism: beauty is a property constructed in the mind, so exists only in descriptions of things. * Platonic realism: beauty is a property that exists in an ideal form independently of any mind or description. * Aristotelian realism: beauty is a property that only exists when beautiful things exist. Taking a broader view, the main positions are generally considered to classifiable as: realism, nominalism, and idealism (sometimes simply called "anti-realism" with regard to universals).〔MacLeod & Rubenstein (2006), §3.〕 Realists posit the existence of independent, abstract universals to account for attribute agreement. Nominalists deny that universals exist, claiming that they are not necessary to explain attribute agreement. Conceptualists posit that universals exist only in the mind, or when conceptualized, denying the independent existence of universals. Complications which arise include the implications of language use and the complexity of relating language to ontology. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Universal (metaphysics)」の詳細全文を読む スポンサード リンク
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