Putnam’s work spans a broad spectrum of philosophical interests, yet nonetheless reflects thematic unity in its concern over the question of realism. The dynamic nature of Putnam's thought manifests itself in his critique and revision of some of his own significant contributions to philosophy, such as the theories of functionalism and quantum logic. The driving force underlying this dynamic is a persistent attempt to defend the notions of truth and objectivity against philosophical positions that discredit them. Beginning his career as a critic of logical positivism, Putnam opposed verificationism and conventionalism, arguing for a realist understanding of scientific theories. He rejected the traditional conception of meaning according to which speaker’ mental states determine meaning and consequently, reference, and put forward a conception of meaning on which external reality, what one talks about, contributes essentially to meaning. Further, citing what he called the division of linguistic labour, Putnam saw the conferring of meaning as a social rather than an individual enterprise. In response to the relativistic challenge that the incommensurability of different theories precludes any possibility of intertheoretical dialogue, Putnam construed reference as relatively insensitive to theoretical variation, so that the continuity and rationality of science and communication are upheld.
The Copenhagen interpretation of quantum mechanics posed yet another difficulty for realism, for it construed quantum states as probability amplitudes rather than real physical states, as is the case in classical mechanics. Putnam initially saw quantum logic as an alternative which was compatible with realism, and argued that logic, like geometry, can be revised on the basis of empirical considerations. Without retracting the general point about the feasibility of a revision of logic, Putnam eventually distanced himself from quantum logic, and pursued other ways of saving realism and solving the notorious measurement problem that plagues the Copenhagen interpretation.
In the philosophy of mind, Putnam proposed functionalism, the view that mental states are characterized by function rather than material constitution. Despite the great impact of functionalism, Putnam came to the conclusion that on their own, functional state are insufficient to characterize mental states. He used his externalist theory of meaning to argue for a broader view of the mental that takes account of the mind's links to external reality and the diverse experiences of different speakers. The externalist theory of meaning thus impacts on other issues central to Putnam's philosophy. Putnam also made substantial contributions to mathematics, including is work on the insolvability of Hilbert’s tenth problem. Hilbert's 1900 list of 23 outstanding problems in mathematics set the agenda for generations of mathematicians so that progress on each one of these problems is a milestone in the history of mathematics. In 1976, Putnam launched an attack on the coherence of the view he termed ’metaphysical realism’. The metaphysical realist maintains there is a single theory that is true of the world and that its truth is absolutely objective, independently of the theory's conceptual frame and its verification procedure. Arguing that relativism and scepticism are disguised forms of metaphysical realism, and likewise incoherent, he suggested an alternative, referred to as ’internal realism’. Internal realism replaced this notion of truth with that of warranted assertability. In so doing it constituted a significant change, for it meant adopting a verificationist theory of meaning, a theory that Putnam had objected to in earlier writings. Within a few years, however, Putnam became dissatisfied with internal realism, surmising that verificationism was incoherent. Although he remained critical of some naïve versions of realism, his later philosophy is undoubtedly realist not only with regard to scientific and mathematical truth, but also with regard to moral values. Articulation of this recovered realism and its viability as an alternative to conventionalist and relativist positions is the focus of many of Putnam’s later writings, as well as much of the criticism they have incurred. Putnam's search for a philosophical position that secures a place for objective truth has always been accompanied by deep moral and social sensibilities. These sensibilities were first expressed in political activity but as of the late 1970s are also manifest in his philosophical writings. Many of Putnam's later titles bear witness to his engagement with moral issues: Meaning and the Moral Sciences (1978), Realism with a Human Face (1990a), Words and Life (1994),The Collapse of the Fact/Value Dichotomy (2002), Ethics Without Ontology (2004) and Jewish Philosophy as a Guide to Life (2008).
Economic Analysis of Law as Implementation of Petrazhycki’s Project for Externalist Theory of Legal Policy