This thesis can be refuted easily, since it destroys its own ontological foundation.
If we assume thinking to be a function of a biological and therefore physical system, we are forced, by this argument, to limit the capacity of thinking to the field of operability of this function within the biologically preset boundaries of the neuro-physiological system called brain. A function that grew by nature's selective evolution must inexorably be bound to the limits of the organic system within which this function is operative. The function is dependent on the system, it cannot transcend the system itself.
The term function may have several connotations:
Definitions (i), (ii) and (iv) in particular contribute to the assumption that a function is embedded within a whole, a system. A function does not possess self-motivation or self-causation. It is strictly subject to natural laws and completely determined by them. Thus if thinking or knowledge is a function of a natural system, it must necessarily obey the laws of that system. This conclusion, however, is absurd: thinking would be completely determined and notions such as "free will" or "responsibility" could not be conceived reasonably. This is brute behaviorism, the human being as a machine.
If the above thetic assumption is true, why is it possible for us, to transcend these functional limits of the system in the very act of thought itself? Although we confess that most people never overcome the natural pragmatism of their thinking, we nevertheless have numerous examples of great human thinkers that excelled mediocrity and thus pragmatism by far. They were capable of extending their capacity of thinking into speculative and mystical realms. Philosophy is no means for survival. Never would a biological system have devised such a useless function. If thinking were only a product of the nature of the brain, it would never be possible within the given laws of nature to transcend the system's functionality. There is enough proof that thought CAN transcend the narrow set of functions of the brain. The reason that we can have thoughts going beyond the biological restraints of our brain, proves the immateriality and independence of our mind from matter.
Mind is not a product of
Neither are they functioning independently of each other. Mind is
independent in so far as it does not depend ontologically on the existence of matter,
but mind needs the brain to express itself through the human body and to give us
an extended set of instruments for living in this world. This set of instruments,
emotions etc. are possibilities
to transcend the biological nature of the human species, to go beyond the state
of animality, to attain the special dignity of a homo sapiens.
 Merriam Webster's Collegiate Dictionary, 10th Edition