First, everything I say presumes the action of the principle of sufficient reason (psr), which states that everything is the way it is for a reason.
Second, we must separate mind from brain. It is clear from the action of the unconscious, the brain, in creating the vivid, detailed, highly structured domain of dreams, that the brain is perfectly capable of highly functioning outside of the conscious mind. We also see this in the careful actions of somnambulists, and the autonomic processes of the body. In other words the brain can function reflexively in all capacities without any need for a conscious mind. In terms of metaphysical naturalism, there is insufficient reason for the conscious mind to exist.
Third, the fact that, as everyone knows from empirical experience, the mind does exist, demonstrates that the mind is distinct from the brain. This mind I call “soul”.