First, the brain is not the measure of the thinking power. The mental capacity of man, and the thinking power of the soul, are not exactly the same thing. The first implies both soul and body, the second regards the soul alone; the first presents to us the musician with his instrument, the second exhibits only the musician himself. The brain is the organ, the soul is the organist. You cannot reasonably pretend that the musical talent, genius, and skill of an organist increase and decrease with the number and quality of the pipes which happen to be in the organ. All you can say is that the musical talent of the organist will have a better chance of a favorable show with a rich rather than with a poor instrument. The organ, therefore, is not the measure of the ability of the organist, and the brain is not the measure of the thinking power. Hence from the fact that the mental capacity of man is enlarged, as you say, in proportion to the material growth of his brain, we have no right to conclude that the thinking principle, the soul, grows with the brain; the right conclusion is that the soul, being in possession of a better instrument, finds itself in better conditions for the exercise of its intrinsic power. The organ is improved and the music is better; but the organist is the same. Secondly, brain and mind are at present necessarily connected. Does it follow that therefore the soul cannot have a separate existence? By no means. If this conclusion were logical, you might on the same ground affirm also that the body cannot have a separate existence; for the body is as necessarily connected with the soul as the soul is with the body. The reason why your conclusion cannot hold is that the connection of body and soul is necessary only inasmuch as both are indispensable for the constitution of the human nature. But the human nature is not immortal; the soul must quit the body when the organism becomes unfit for the operations of animal life; and therefore the connection of the soul with the body is not absolutely, but only hypothetically, necessary. The soul has its own existence distinct from the existence of the body, for the soul is a substance no less than the body; and therefore it is no less competent to have a separate existence. You deny, I know, that the soul is a substance distinct from the body; but what is the weight of such a denial? What you speculatively deny in your book, you practically admit in the secret of your conscience whenever you say _I am_. It is not the body that says _I_; it is the soul: and it is not an accident that perceives self; it is a substance.