Putnam's best-known work concerns philosophy of mind. His most noted original contributions to that field came in several key papers published in the late 1960s that set out the hypothesis of multiple realizability. In these papers, Putnam argues that, contrary to the famous claim of the type-identity theory, it is not necessarily true that "Pain is identical to C-fibre firing." According to Putnam's papers, pain may correspond to utterly different physical states of the nervous system in different organisms even if they all experience the same mental state of "being in pain".