Armstrong David M. (b.1926), Australian philosopher of mind and metaphysician, and until his retirement Challis Professor of Philosophy at Sydney, noted for his allegiance to a physicalist account of consciousness and to a realist view of properties conceived as universals. A Materialist Theory of the Mind (1968) develops a scientifically motivated version of the view that mental states are identical with physical states of the central nervous system. Universals and Scientific Realism (1978) and What Is a Law of Nature? (1983) argue that a scientifically adequate ontology must include universals in order to explain the status of natural laws. Armstrong contends that laws must be construed as expressing relations of necessitation between universals rather than mere regularities among particulars. However, he is only prepared to acknowledge the existence of such universals as are required for the purposes of scientific explanation. Moreover, he adopts an ‘immanent’ or ‘Aristotelian’ (as opposed to a ‘transcendent’ or ‘Platonic’) realism, refusing to accept the existence of uninstantiated universals and denying that universals somehow exist ‘outside’ space and time. More recently, Armstrong has integrated his scientifically inspired physicalism and property realism within the overall framework of an ontology of states of affairs, notably in A World of States of Affairs (1997). Here he advocates the truthmaker principle that every truth must be made true by some existing state of affairs and contends that states of affairs, rather than the universals and particulars that he regards as their constituents, are the basic building blocks of reality. Within this ontology, which in some ways resembles that of Wittgenstein’s Tractatus, necessity and possibility are accommodated by appeal to combinatorial principles. As Armstrong explains in A Combinatorial Theory of Possibility (1989), this approach offers an ontologically economical alternative to the realist conception of possible worlds defended by David Lewis. See also LAWLIKE GENERALIZATION, META- PHYSICAL REALISM , PHILOSOPHY OF SCIENCE , SCIENTIFIC REALIS. E.J.L.