Contemporary analytic philosophers of mind generally use the term “belief” to refer to the attitude we have, roughly, whenever we take something to be the case or regard it as true. To believe something, in this sense, needn't involve actively reflecting on it: Of the vast number of things ordinary adults believe, only a few can be at the fore of the mind at any single time. Nor does the term “belief”, in standard philosophical usage, imply any uncertainty or any extended reflection about the matter in question (as it sometimes does in ordinary English usage). Many of the things we believe, in the relevant sense, are quite mundane: that we have heads, that it's the 21st century, that a coffee mug is on the desk. Forming beliefs is thus one of the most basic and important features of the mind, and the concept of belief plays a crucial role in both philosophy of mind and epistemology. The “mind-body problem”, for example, so central to philosophy of mind, is in part the question of whether and how a purely physical organism can have beliefs. Much of epistemology revolves around questions about when and how our beliefs are justified or qualify as knowledge.
It is common to think of believing as involving entities beliefs that are in some sense contained in the mind. When someone learns a particular fact, for example, when Kai reads that astronomers no longer classify Pluto as a planet, he acquires a new belief (in this case, the belief that astronomers no longer classify Pluto as a planet). The fact in question or more accurately, a representation, symbol, or marker of that fact may be stored in memory and accessed or recalled when necessary. In one way of speaking, the belief just is the fact or proposition represented, or the particular stored token of that fact or proposition; in another way of speaking, the more standard in philosophical discussion, the belief is the state of having such a fact or representation stored. (Despite the ease with which we slide between these different ways of speaking, they are importantly distinct: Contrast the state of having hot water in one's water heater the state of being “hot-water ready”, say with the stuff actually contained in the heater, that particular mass of water, or water in general.)