Metaphilosophy and Ontology
In the late 1970s and the 1980s, stimulated by results from mathematical logic and by some ideas of Quine, Putnam abandoned his long-standing defence of metaphysical realism—the view that the categories and structures of the external world are both causally and ontologically independent of the conceptualizations of the human mind. He adopted a rather different view, which he called "internal realism".
Putnam renounced internal realism in his reply to Simon Blackburn in the volume Reading Putnam. The reasons he gave up his "antirealism" are stated in the first three of his replies in "The Philosophy of Hilary Putnam", an issue of the journal Philosophical Topics, where he gives a history of his use(s) of the term "internal realism", and, at more length, in his The Threefold Cord: Mind, Body and World (1999).
Internal realism was the view that, although the world may be causally independent of the human mind, the structure of the world—its division into kinds, individuals and categories—is a function of the human mind, and hence the world is not ontologically independent. The general idea is influenced by Kant's idea of the dependence of our knowledge of the world on the "categories of thought".
The problem with metaphysical realism, according to Putnam, was that it fails to explain the possibility of reference and truth. According to the metaphysical realist, our concepts and categories refer because they match up in some mysterious manner with the pre-structured categories, kinds and individuals that are inherent in the external world. But how is it possible that the world "carves up" into certain structures and categories, the mind carves up the world into its own categories and structures, and the two "carvings" perfectly coincide? The answer must be that the world does not come pre-structured but that structure must be imposed on it by the human mind and its conceptual schemes. In Reason, Truth, and History, Putnam identified truth with what he termed "idealized rational acceptability." The theory, which owes something to C.S. Peirce, is that a belief is true if it would be accepted by anyone under ideal epistemic conditions.
Nelson Goodman had formulated a similar notion in Fact, Fiction and Forecast in 1956. In that work, Goodman went as far as to suggest that there is "no one world, but many worlds, each created by the human mind." Putnam rejected this form of social constructivism, but retained the idea that there can be many correct descriptions of reality. No one of these descriptions can be scientifically proven to be the "one, true" description of the world. This does not imply relativism, for Putnam, because not all descriptions are equally correct and the ones that are correct are not determined subjectively.
Under the influence of C.S. Peirce and William James, Putnam also became convinced that there is no fact–value dichotomy; that is, ethical and aesthetic judgments often have a factual basis, while scientific judgments have an ethical element.
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