In recent times, physiology has made great and, to many, startling advances in the direction of connecting mental phenomena with brain-changes. Physiologists have established the correlation of vital with chemical and physical forces,[40] and probably in some sense, at least, of mental with vital forces. They have proved, in every act of perception, first a physical change in a nerve-terminal, then a propagated thrill along a nerve-fiber, and then a resulting change, physical or chemical, in the brain; and in every act of volition, a change first in a brain-cell, then, a return thrill along a nerve-fiber, and a resulting contraction of a muscle. Even the velocity of the transmission to and fro has been measured, and the time necessary to produce brain-changes estimated. They have also established the existence of physical and chemical changes in the brain corresponding to every change of mental state, and with great probability an exact quantitative relation between these changes of brain and the corresponding changes of mind. In the near future they may do more: they may localize all the different faculties and powers of the mind, each in its several place in the brain, and thus lay the foundations of a truly scientific phrenology. In the far-distant future we may possibly do much more. We may connect each kind of mental state with a different and distinctive kind of brain-change. We may find, for example, a right-handed rotation of atoms associated with _love_, and a left-handed rotation associated with _hate_, or a gentle sideways oscillation associated with _consciousness_, and a vertical pounding associated with _will_. Now, suppose all this, and even much more, be done in the way of associating, both in degree and in kind, mental changes with brain-changes. What then? “Why,” say the materialists, “we thereby identify _mind_ with _matter_, mental forces with material forces. Thought, emotion, consciousness and will become products of the brain, in the same sense as bile is a product of the liver, or urea a product of the kidneys.”