mind-body problem
Most generally, the problem of describing the relationship between the mind and body (or brain). First explicitly raised by Descartes, it is, perhaps, the best know problem in the philosophy of mind. See dualism, epiphenomenalism, monism, and materialism.

Perhaps the oldest problem in the philosophy of mind, the mind-body problem dates back at least to Plato. By some counts, Plato was the first dualist, with the first materialist, Aristotle, close at hand. Plato contends that the soul is distinct from the body and is capable of maintaining a separate existence from it. Aristotle, in contrast, feels that body and soul are two aspects of the same underlying substance (form and matter). It should be noted that it is by no means unanimous that Aristotle was not a dualist.
Dualism has been the driving force behind the existence of the mind-body problem and has been by far the majority view until recently. Partially due to the influence of Descartes, the dualist position has reigned supreme. However, by espousing a distinct type of substance for the mind, dualists invite the question: What is it that makes it possible for two contraries (one spatially existing and the other not) to interact as our minds seem to with our brains?
In attempting to answer this question, Descartes claimed that the pineal gland was the interface between the mind and the rest of the brain; he considered it the seat of the soul. The causal interactions between mind and brain are two-way. In perception, the physical states of the world influence our bodies which influence our brains which, via the pineal gland, influence our soul. The reverse is true for deliberate action. However, even on this account, it remains a mystery how states of the non-spatial soul (or mind) are to causally interact with the states of the spatial brain.
Some ways of answering, or avoiding, this difficulty fall under the names of epiphenomenalism, monism, and materialism.
Chris Eliasmith