Identity is perhaps the simplest of relations and yet it is at the centre of many philosophical controversies. For example, are mental states identical to physical states? The mind–brain identity theory holds that the mind is identical to the brain, at least in the sense that all mental states are identical to brain states. This theory depends, in part, on a certain view about the nature of identity; namely, that some identity statements express contingent truths or what are sometimes called contingent identities. Independent of the resolution of the mind–body problem, this view of identity has had wide support, in part because of the tradition that the distinction between contingent truths and necessary truths is equivalent to the distinction between analytic truths and synthetic truths (see pp. 3–6.). So, if a certain claim (such as brain state B is identical to mental state M) is not analytic, it must be contingent. This is acceptable to many philosophers because many identity statements can only be discovered empirically. Prior to investigation, it was not known that the morning star is, in fact, the evening star; knowledge that the morning star is the evening star was acquired though observation and experience. This is called a posteriori knowledge, to be contrasted with a priori knowledge, which knowledge is obtained from reason and logic alone. The claim that B is identical to M even if it is true is one that must be discovered through empirical means and hence it is not an example of a priori knowledge.