Now nothing can be more certain respecting the thinking principle, than its individuality. It has been said, that the mind can entertain but one thought at one time; and certain it is, from the nature of attention, and from the association of ideas, that unity is one of the principal characteristics of mind. It is this which constitutes personal identity; an attribute that, however unsatisfactory may be the explanations which have been given respecting it, we all of us feel, and that lies at the foundation of all our voluntary actions, and all our morality. Analogous to this unity of thought and mind, is the arrangement of the nerves and the brain in the human body. The nerves all lead up to the brain; and there is a centrical point in the brain itself, in which the reports of the senses terminate, and at which the action of the will may be conceived to begin. This, in the language of our fathers, was called the "seat of the soul." We may therefore, without departing from the limits of a due caution and modesty, consider this as the throne before which the mind holds its court. Hither the senses bring in their reports, and hence the sovereign will issues his commands. The whole system appears to be conducted through the instrumentality of the nerves, along whose subtle texture the feelings and impressions are propagated. Between the reports of the senses and the commands of the will, intervenes that which is emphatically the office of the mind, comprising meditation, reflection, inference and judgment. How these functions are performed we know not; but it is reasonable to believe that the substance of the brain or of some part of the brain is implicated in them. Still however we must not lose sight of what has been already said, that in the action of the mind unity is an indispensible condition. Our thoughts can only hold their council and form their decrees in a very limited region. This is their retreat and strong hold; and the special use and functions of the remoter parts of the brain we are unable to determine; so utterly obscure and undefined is our present knowledge of the great ligament which binds together the body and the thinking principle.