can be many correct descriptions of reality. None of these descriptions can be scientifically proven to be the "one, true" description of the world. He thus accepted "conceptual relativity"—the view that it may be a matter of choice or convention, e.g., whether mereological sums exist, or whether spacetime points are individuals or mere limits. Curtis Brown has criticized Putnam's internal realism as a disguised form of subjective idealism, in which case it is subject to the traditional arguments against that position. In particular, it falls into the trap of solipsism. That is, if existence depends on experience, as subjective idealism maintains, and if one's consciousness ceased to exist, then the rest of the universe would also cease to exist. In his reply to Simon Blackburn in the volume Reading Putnam, Putnam renounced internal realism because it assumed a "cognitive interface" model of the relation between the mind and the world. Under the increasing influence of William James and the pragmatists, he adopted a direct realist view of this relation. Although he abandoned internal realism, Putnam still resisted the idea that any given thing or system of things can be described in exactly one complete and correct way. He came to accept metaphysical realism in a broader sense, rejecting all forms of verificationism and all talk of our "making" the world. In the philosophy of perception, Putnam came to endorse direct realism, according to which perceptual experiences directly present one with the external world. He once further held that there are no mental representations, sense data, or other intermediaries between the mind and the world. By 2012, however, he rejected this commitment in favor of "transactionalism", a view that accepts both that perceptual experiences are world-involving transactions, and that these transactions are functionally describable (provided that worldly items and intentional states may be referred to in the specification of the function). Such transactions can further involve qualia. Quantum mechanics During his career, Putnam espoused various positions on the interpretation of quantum mechanics. In the 1960s and 1970s, he contributed to the quantum logic tradition, holding that the way to resolve quantum theory's apparent paradoxes is to modify the logical rules by which propositions' truth values are deduced. Putnam's first foray into this topic was "A Philosopher Looks at Quantum Mechanics" in 1965, followed by his 1969 essay "Is Logic Empirical?". He advanced different versions of quantum logic over the years, and