The function of the brain is to limit our mental life to what is practically useful.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

““which is the lowest degree of mind—mind without memory—is really part of matter, as we understand matter.” Pure perception is constituted by dawning action, its actuality lies in its activity. It is in this way that the brain becomes relevant to perception, for the brain is not an instrument of action. The function of the brain is to limit our mental life to what is practically useful. But for the brain, one gathers, everything would be perceived, but in fact we only perceive what interests us. “The body, always turned towards action, has for its essential function to limit, with a view to action, the life of the spirit.”” source