The mind-brain identity theory is thus a theoretical expression of the unique relation between the undergoing or having and the undergone or had. An important objection to identity theory (IT) has been raised in the form of a requirement to explain how two things that are supposed to be identical in some very strong sense amounting to necessity are nevertheless commonly thought of as only contingently identical. The putative identity can be further elucidated by considering in turn: the terms; the identity sign; the identity proposition involved in statements of the theory. Mental–physical Isomorphism is generally agreed that, if it were shown that a brain state were associated with more than one mental state, then IT would be falsified, for of course a one–many physical–mental relation would rule out identity. However, there remains the substantial objection that, in point of fact, the mental and physical realms are logically incommensurable; they are not 'made for one another'.