A universal is a property or relation that can be instanced, or instantiated, by a number of different particular things: each yellow thing provides an instance of the property of yellowness, and each square thing the property of being square. The things covered by a universal are thus similar in some respect. The general questions asked about universals include: are they discovered or invented? How are we to think of something that has itself no spatial position, yet is instanced at many places and times? What is the relation of instantiation? Can sharing the same property be analysed in terms of resemblance? How does the mind perceive the general property as well as the particular instance of it? Approaches to universals include Platonism, or the position that universals exist independently of things (ante rem); the Aristotelian belief that universals exist in things (in re) but not independently of them; conceptualism, or the view that they are reflections of the propensity of the mind to group things together (post rem, or abstracted from things); nominalism, or the view that the universal is the breath of the voice (flatus vocis), i.e. that to share a universal is simply to be describable by the same word; and finally a general suspicion that the whole issue is the result of a misleading reification, trapping us into thinking of two categories of thing (the particular and the universals it instances) instead of just particulars. However, a theory of universals is vital in many areas: for example, one's attitude to knowledge and science will depend upon whether natural kinds are thought of as invented or discovered, and the problem of induction is made even less tractable if the similarities we project are thought of as having only a conventional or nominalistic status. The problem of universals was a major topic of controversy in medieval philosophy: see Boethius, Ockham, Porphyry. See also forms.