Part of the puzzlement here is because of the limitations of imagination: influenced by his Princeton colleague, Saul Kripke, Nagel believes that any type identity statement that identifies a physical state type with a mental state type would be, if true, necessarily true. But Kripke argues that one can easily imagine a situation where, for example, one's C-fibres are stimulated but one is not in pain and so refute any such psychophysical identity from the armchair. (A parallel argument does not hold for genuine theoretical identities.) This argument that there will always be an explanatory gap between an identification of a state in mental and physical terms is compounded, Nagel argues, by the fact that imagination operates in two distinct ways. When asked to imagine "sensorily", one imagines C-fibres being stimulated; if asked to imagine "sympathetically", one puts oneself in a conscious state resembling pain. These two ways of imagining the two terms of the identity statement are so different that there will always seem to be an explanatory gap, whether or not this is the case. (Some philosophers of mind have taken these arguments as helpful for physicalism on the grounds that it exposes a limitation that makes the existence of an explanatory gap seem compelling, while others have argued that this makes the case for physicalism even more impossible as it cannot be defended even in principle.)