The mind-body problem concerns the explanation of the relationship, if any, that exists between minds, or mental processes, and bodily states or processes. One of the aims of philosophers who work in this area is to explain how a non-material mind can influence a material body and vice versa. However, this is not uncontroversial or unproblematic, and adopting it as a solution raises questions. Perceptual experiences depend on stimuli which arrive at various sensory organs from the external world and these stimuli cause changes in mental states; ultimately causing sensation. A desire for food, for example, will tend to cause a person to move their body in a manner and in a direction to obtain food. The question, then, is how it can be possible for conscious experiences to arise out of an organ (the human brain) possessing electrochemical properties. A related problem is to explain how propositional attitudes (e.g. beliefs and desires) can cause neurons of the brain to fire and muscles to contract in the correct manner. These comprise some of the puzzles that have confronted epistemologists and philosophers of mind from at least the time of René Descartes. John Locke considered personal identity (or the self) to be founded on consciousness (viz. memory), and not on the substance of either the soul or the body. Book II Chapter XXVII entitled "On Identity and Diversity" in "An Essay Concerning Human Understanding" (1689) has been said to be one of the first modern conceptualizations of consciousness as the repeated self-identification of oneself. Through this identification, moral responsibility could be attributed to the subject and punishment and guilt could be justified, as critics such as Nietzsche would point out. According to Locke, personal identity (the self) "depends on consciousness, not on substance" nor on the soul. We are the same person to the extent that we are conscious of the past and future thoughts and actions in the same way as we are conscious of present thoughts and actions. If consciousness is this "thought" which "goes along with the substance [...] which makes the same person", then personal identity is only founded on the repeated act of consciousness: "This may show us wherein personal identity consists: not in the identity of substance, but [...] in the identity of consciousness". For example, one may claim to be a reincarnation of Plato, therefore having the same soul substance. However, one would be the same person as Plato only if one had the same consciousness of Plato's thoughts and actions that he himself did.