Medicine can sometimes be a lot like auto mechanics. If you listen to master mechanics like the guys on Car Talk, you can almost hear the wheels turn as they ask probing questions to narrow the possible explanations of the caller's car problem to a narrow differential diagnosis and, often by this application of pure reason and experience, the solution to the problem. It is the same kind of reasoning process, albeit a million times funnier, that you would hear if you could listen in on a doctor's ruminations about a patient's illness.
You might doubt that we are able to apply this kind of thinking to the mind, but I propose that you can think mechanistically about mind if you focus not on the parts, but on what they do; that is, on their function. In this sense, mind is the organic function of the brain, the way circulation is the function of the organ we call the heart. Whereas the heart pumps, the mind is there, fundamentally, to enhance our acquisition and use of energy from the environment.
All living organisms depend on energy to survive. It stands to reason, then, that a well adapted organism is one that can acquire energy reliably. Simpler organisms do so by trial and error, which may be a successful approach for a species, but a raw deal for the individual organism that gets caught without resources in a hostile environment.
The more highly developed the organism, the more wasteful is a trial and error approach to individual survival. Complex organisms have evolved means to detect sources of stored energy in the environment. The brain helps the organism to detect and acquire energy, and to retain information about how to obtain it in the future.
A brain, being highly organized, requires a huge investment of energy. Therefore most creatures with brains also have evolved the capacity to defend themselves from harm. Brainy creatures that communicate and cooperate with one another have additional advantages. Ants secrete pheromones and bees dance to inform colony members and hive mates where to find food sources. We humans can condense our own internal experiences--perceptions, thoughts, feelings, intentions, memories, beliefs--into time- and energy-efficient signs and symbols. We can pass along to others not only information about sources of energy and danger, but also precise information about the qualities of the objects we have encountered.
The ability to communicate about internal states and the qualities of objects has contributed to the development of social cooperation. We can tell others what we need and learn what they need. Information about thoughts, feelings, and intentions also adds depth to information about the world. When we communicate our subjective reflections on the objective world we provide richer and more nuanced information, and reduce trial and error for others. Symbolic communication--language--has made it possible to transmit information over time and space, so that people we never even meet help us survive and prosper from the experiences they had long ago and far away.
A brain allows an organism to adapt its actions to its environment; it optimizes the search for energy and defends its adaptations. A brain that is equipped to communicate and cooperate with others optimizes resources and defense for others, and gains in return. A brain that can detect, retain, recall, and communicate efficiently about remote and internal events uses a special set of functions that occur abundantly only in humans, and partially in only a few other mammals: these are called mental functions, or mind. When we describe someone as "mentally ill" or "out of her or his mind" or behaving "mindlessly," we mean that the person's problem is abnormal or inappropriate or inadequate or painful or erroneous thoughts, feelings, and intentional actions.