this is just as incomprehensible and metaphysical, though it often assumed a physical appearance. The conception of the soul as a "substance" is far from clear in many psychologists; sometimes it is regarded as an "immaterial" entity of a peculiar character in an abstract and idealistic sense, sometimes in a concrete and realistic sense, and sometimes as a confused _tertium quid_ between the two. If we adhere to the monistic idea of substance, which we develop in chap. xii., and which takes it to be the simplest element of our whole world-system, we find _energy_ and _matter_ inseparably associated in it. We must, therefore, distinguish in the "substance of the soul" the characteristic psychic _energy_ which is all we perceive (sensation, presentation, volition, etc.), and the psychic _matter_, which is the inseparable basis of its activity--that is, the living protoplasm. Thus, in the higher animals the "matter" of the soul is a part of the nervous system; in the lower nerveless animals and plants it is a part of their multicellular protoplasmic body; and in the unicellular protists it is a part of their protoplasmic cell-body. In this way we are brought once more to the psychic organs, and to an appreciation of the fact that these material organs are indispensable for the action of the soul; but the soul itself is _actual_--it is the sum-total of their physiological functions. However, the idea of a specific "soul-substance" found in the dualistic philosophers who admit such a thing is very different from this. They conceive the immortal soul to be material, yet invisible, and essentially different from the visible body which it inhabits. Thus _invisibility_ comes to be regarded as a most important attribute of the soul. Some, in fact, compare the soul with ether, and regard it, like ether, as an extremely subtle, light, and highly elastic material, an imponderable agency, that fills the intervals between the ponderable particles of the living organism, others compare the soul with the wind, and so give it a gaseous nature; and it is this simile which first found favor with primitive peoples, and led in time to the familiar dualistic conception. When a man died, the body remained as a lifeless corpse, but the immortal soul "flew out of it with the last breath."