Beginning in the mid-1970s, Putnam sought to distinguish his understanding of realism from what he now called “metaphysical realism.” According to Putnam (“Why There Isn’t a Ready-Made World” [1983]),

What the metaphysical realist holds is that we can think and talk about things as they are, independently of our minds, and that we can do this by virtue of a “correspondence” relation between the terms in our language and some sorts of mind-independent entities.

For Putnam, this picture of word-world correspondence is absurd, pointing to a realism gone wild. Putnam considered metaphysical realism to be blind to the autonomy and complexity of human language. In particular, it is blind to the fact that the same reality can be described in multiple ways.

Whether Putnam’s early realism was ever “metaphysical” in this sense is questionable, but it is clear that his explicit critique of metaphysical realism gives Putnam’s philosophy a Kantian bent; this is particularly salient in the papers collected in Reason, Truth and History (1981). Much like Kant’s denial of the knowability of the “thing in itself,” the modest realism Putnam endorsed there eschewed the notion of reality “in itself,” with its built-in representation. After initially calling his position “internal realism,” Putnam later referred to it as “commonsense realism” or simply “realism,” as opposed to “Realism.” The essential point is that none of these changes impelled Putnam to deny objective truth. Truth under a description, he maintained, is all the truth one needs to avoid subjectivism and relativism. He emphasized, for example, the prevalence in science and mathematics of the phenomenon of equivalence between different theories or descriptions. Such equivalent theories can differ in their respective ontologies (e.g., one adducing forces, the other fields) and still predict and explain the very same phenomena.

Not surprisingly, this middle ground soon came under fire. Critics considered Putnam’s realism unstable and at risk of collapsing into either metaphysical realism or relativism. The latter option was particularly repugnant to Putnam: in its thoroughgoing denial of objective truth, relativism is but a form of radical skepticism and is clearly at odds with Putnam’s realism.

One of Putnam’s striking insights at that time was that the two polar positions—metaphysical realism on the one hand and skepticism on the other—are equally vulnerable, and for the same reason. Both positions construe truth as radically non-epistemic, and, thus, both countenance the possibility that the best scientific theory of the world—a theory that satisfies every epistemic desideratum and is perfect in every methodological and aesthetic respect—could still turn out to be false. But this possibility, Putnam argued, is meaningless, and so are the metaphysical views that countenance it.

Putnam devoted considerable effort to the rebuttal of skepticism. In particular, he addressed the oft-touted skeptical claim that, for all one knows, one might be a brain in a vat. As Putnam described it in “Brains in a Vat” (1981), this thought experiment contemplates the following scenario:

A human being (you can imagine this to be yourself) has been subjected to an operation by an evil scientist. The person’s brain (your brain) has been removed from the body and placed in a vat of nutrients which keeps the brain alive. The nerve endings have been connected to a super-scientific computer which causes the person whose brain it is to have the illusion that everything is perfectly normal.

The purpose of the thought experiment, Putnam noted, is “to raise the classical problem of scepticism with respect to the external world in a modern way. (How do you know you aren’t in this predicament?)” Here too, Putnam’s argument follows directly from his theory of meaning. On the externalist conception of meaning, words in the vat brain language do not have the same referents as “normal” words in “normal” human languages because they are not causally connected in the normal way to the referents of normal words. In particular, for brains in a vat, the word tree would refer not to real trees but at best to images of trees produced by a supercomputer and experienced by envatted brains. Likewise, the word vat in the vat brain language would refer not to real vats but at best to images of vats so produced and experienced. “In short,” Putnam concluded, “if we are brains in a vat, then ‘We are brains in a vat’ is false.” The brains-in-a-vat hypothesis is thus paradoxical and self-defeating. Skepticism is, it turns out, fundamentally flawed: the skeptic’s concerns cannot even be expressed in a meaningful way.

Putnam’s realism also led him to pursue a realist interpretation of quantum mechanics, a theory generally considered to pose insurmountable difficulties for the realist. Many physicists believe, for example, that quantum mechanics, unlike classical mechanics, does not represent the actual physical state of a system. Rather, it is an algorithm for calculating the probabilities of the results of measurements. Putnam’s first attempt to provide a more realist understanding of quantum mechanics invoked the radical claim that logic is empirical. He argued that quantum mechanics does, in fact, represent real physical states, but the logical rules used for quantum calculations diverge from those used in classical physics. Putnam later renounced that approach, adopting a less radical version of quantum mechanics put forward by David Bohm and further developed by G.C. Girardi, A. Rimini, and T. Weber.

Putnam’s realism also extended to mathematics. Together with the American philosopher W.V.O. Quine, he proposed the indispensability thesis: given that mathematical objects such as numbers, sets, and groups play an indispensable role in the best theories of the world, their reality must be granted (see also philosophy of mathematics: The Fregean argument for Platonism).