The mind-body problem is an issue that yields various philosophical ideas. Functionalism is one such view that proposes a solution. The ideals of functionalism define mental events as brain states, and that mental states are created through a relationship with each other as well as with physical stimuli (such as the senses) teamed with a physical or behavioral result or response.
Functionalism holds that the knowledge of the physical (or the functional methods) are not the complete picture. It is more concerned with the relation to similar and their opposite reaction, and how they relate to other functions. According to this view, mental states can only be seen through their relations with an input, the brain state that results or is created, and after the physical or observable affects of behavior (seen from an outside perspective). It sometimes helps to look at the situation in reverse to see the specificity of it all.
For example, we know that a certain mental state, though mostly identifiable, cannot reveal the physical process or component that creates it, so a mental state must contain an element that is not reducible completely to the physical.
Functionalism is often argued and compared against behaviorism and identity theories in the philosophical realm. Mainly, functionalism can be contrasted to the two theories by it's description of mental events. Both theories focus mainly on the sensory stimuli and behavioral result relationship, however, functionalism also takes into account other mental states (with little or no direct input), most arguably states like depression or stress. A major difference between Functionalism and identity theories is that with identity theory, mental states are confined to brain activity. Behaviorism on the other hand, reduces mental events and activity entirely based on an input/output relationship. But therein lies a problem in behaviorism's simplistic view.