Plotinus was-a man of mystical tendencies, but he was both learned and acute. He insists that the soul is an immaterial substance, and he tries to give us a notion of the way in which such a thing can be related to the body. To put it into the body, as Epicurus or Lucretius did, would be to deny its immateriality. This he can not do. To deny that it is related to the body at all is too much even for a philosopher.
In his perplexity he follows a middle course. He tells us that the soul is not in space and is not in things, in the strict sense. But in a certain sense it is in things, or is present to things. It is as a whole in the whole body, and is at the same time wholly in every part of the body; and is, thus, at once divisible and indivisible.
One may legitimately object to this curious doctrine, and criticize Plotinus as giving with one hand what he takes away with the other. It is easy to see what he tried to do, and what he actually did do. He tried to draw a clear distinction between mental phenomena and physical, and to tell us how they are related. He succeeded only in making of the soul an inconsistently material thing, existing in space in an inconceivable way.
But it will not do to treat Plotinus with contempt, and to pass over his doctrine as insignificant. He made an earnest attempt to draw a line between the mental and the physical—surely some such line ought to be drawn—and his influence upon men's minds has been enormous. His doctrine was taken up by Augustine, from whom it passed to the philosophers of the middle ages; and it came ultimately, after undergoing various modifications, to the modern philosophers. Distinct traces of it are to be found in some of the psychologies written at the present day and used in our colleges.
In the seventeenth century that remarkable man Descartes arrived at a fairly clear comprehension of the mechanism of the human body, and of the significance in it of the brain and the nerves. He concluded that the soul or mind has its 'chief seat' in the pineal gland in the brain, and that messages are carried to it from the various parts of the body. Yet he never ventured to put the soul quite frankly and unequivocally in the pineal gland. He still held that the soul was united to all the parts of the body 'conjointly'—the old Plotinic notion.
In other words, he did not go back to Lucretius, and he did not go forward to a clear distinction between mind and body. He remained halting in indecision; he left a dark place for his successors to illuminate with such light as they could furnish. They have been at the work ever since, and have had varying degrees of success.
Now the speculations of the philosophers, especially when they touch upon those things which are supposed to be of great moment to