The extension of the correlation of force to mind, if at all competent, must be made through the nerve-force, a genuine member of the correlated group. Very serious difficulties beset the proposal, but they are not insuperable. The history of the doctrines relating to mind, as connected with body, is in the highest degree curious and instructive, but, for the purpose of the present paper, we shall notice only certain leading stages of the speculation.[18] Not the least important position is the Aristotelian; a position in some respects sounder than what followed and grew out of it. In Aristotle, we have a kind of gradation from the life of plants to the highest form of human intelligence. In the following diagram, the continuous lines may represent the material substance, and the dotted lines the immaterial: A. _Soul of Plants._ ---- Without consciousness. B. _Animal Soul._ ---- Body and mind inseparable. .... C. _Human Soul_--NOUS--_Intellect_. I. Passive intellect. ---- Body and mind inseparable. .... II. Active intellect--cognition of the highest principles. .... Pure form; detached from matter; the prime mover of all; immortal. All the phases of life and mind are inseparably interwoven with the body (which inseparability is Aristotle’s definition of the soul) except the last, the active _nous_, or intellect, which is detached from corporeal matter, self-subsisting, the essence of Deity, and an immortal substance, although the immortality is not personal to the individual. (The immateriality of this higher intellectual agent was net, however, that thorough-going negation of all material attributes which we now understand by the word “immaterial.”) How such a self-subsisting and purely spiritual soul could hold communication with the body-leagued souls, Aristotle was at a loss to say--the difficulty reappeared after him, and has never been got over. That there should be an agency totally apart from, and entirely transcending, any known powers of inert matter, involves no difficulty--for who is to limit the possibilities of existence? The perplexity arises only when this radically new and superior principle is made to be, as it were, off and on with the material principle; performing some of its functions in pure isolation, and others of an analogous kind by the aid of the lower principle. The difference between the active and the passive reason of Aristotle is a mere difference of gradation; the supporting agencies assumed by him are a total contrast in kind--wide as the poles asunder. There is no breach of continuity in the phenomena, there is an impassable chasm between their respective foundations.