As to the soul itself, Kant held that the soul is not a real, but only a logical substance. The Pantheists, Transcendentalists, and Neo-Hegelians try to identify the soul with the divine consciousness. The Associationists (Hume, Davis, Höffding, Sully) say that the soul is a mere group of sensations. The Agnostics and Positivists (Locke, Herbert Spencer, James, Comte) write volume after volume on the soul to prove that they know nothing about it. Then the Materialists assert that there is no soul of any kind; that we secrete thought as a mule secretes sweat. Yet the vital operations of man are inexplicable as resultants of the physical and chemical properties of matter. There is an intrinsic energy that unifies the actions of man, directs processes, controls the tendency of organic matter to pass into the fixity of the inorganic, and effects metabolism. This intrinsic energy is the entelechy, substantial form, or what is popularly called the soul.
In any organic body there is a formal principle. We know that there are activities that proceed from organic bodies, and a formal principle of such activity is a substantial entity whence the organism derives basically its own kind of action, which determines and orders the activity. There are acts of perception in animals such that an external object becomes so internal to the organism of these animals that it is known by one expressed and immanent image, not only as something objectively existing but as good or hurtful to the perceiving animal. The innate and elicited appetites by which the animal tends toward or away from the object are recognized, as are the spontaneous motions which are directed by that knowledge. There must be a principle whence these actions proceed, and this is either an accident of matter or something substantial. It is not an accident of matter, because action can never arise from an accident; it must proceed from a substance. If you say this principle whence these actions arise is not an accident of matter, but matter itself, you would have an extended, composite, inert mass acting; but even if such thing could act, it could never effect a simple immanent image of an object or group of objects external to itself.
No mere machine can build up itself, can make any remote approach to metabolism as an organized body can; and the principle of this immanent action is not matter itself, because it uses, makes, subordinates matter to itself. That principle is positively one, not one by continuity as matter is. Matter as in a crystal grows by mere aggregation, an organism grows by assimilation; a crystal loses force in formation and growth, an organism accumulates force.
The theory that denies the existence of this formal principle does not explain the phenomena of life in organic beings. Uniformity of tendency toward an end is not a characteristic of mere matter; neither is a harmonious interaction of parts, nor the dependence of parts on the unit, nor motion, nor the reproduction of the species.
Moreover, most of the greatest physical scientists strongly maintain that there must be a formal substantial principle in all living things. Among these are Wallace, Nägeli, Askenasy, Preyer, Fechner, Agassiz, von Baer, E. de Beaumont, Blanchard, A. Braun, Brongniart, Bronn, Burmeister, Delff, Milne-Edwardes, Flourens, Goeppert, Griesbach, Heer, Koelliker, Mivart, Quatrefages, Quenstedt, Spiers, Volger, R. Wagner, Liebig, and Joseph Hyrtl.
The formal principle which coexists with matter in the organic body is really though not perfectly distinguished from matter. A formal principle which is necessary for sensation should be either perfectly simple, or at the least so one that its parts together make up one essence: matter, however, cannot have such unity, and as a consequence the formal principle must be distinct from matter. Anything is like its operation, and the parts of any sensitive activity always result in an activity that is essentially one. If we touch a table, by that single touch we at once know that the object is one, wooden, hard, angular, smooth, extended, and so on, and we also know that one subject perceives all these varied qualities. One eye can convey knowledge at once of a thousand objects miles apart, and these objects can be brought into one perception only by a simple subject. An extended complex subject like matter would get one impression (if it could perceive any impression) on one side, one on another, and so on, but it could not unite these.
The formal principle which is in organic bodies is a true substantial form, actuating the body both as to its nature and substance. Together with the body, this principle makes a being one in itself, such that the matter and the form separably are incomplete as regards operation and being. Now, a form is that principle through which anything is established in its own species; light, for example, is the form of a luminous body, heat of a hot substance. A body, however, is established in the human species by receiving a rational soul, and this soul, then, is its form. It is also a substantial form because the soul itself is a substance, not an accident dependent upon another subject. Moreover, from its union with the body another substance—man—arises, and not a thing added to a substance. Man's body is alive, therefore it is a living substance; but life in its secondary actuality is an operation; in its primary actuality it is an essence. The body is made a living substance, not from itself, but from the soul which is added to it. When the soul departs the body is no longer alive. Now, a principle which by a communication of itself determines the body in its essence and differentiates it as a living substance from everything else, is a substantial form. A substantial form, then, or a soul, exists.
The soul, however, must have disposed matter for most of its operations; it cannot exist as a substantial form bombinans in vacuo; but it does not need a human organism complete in all its parts as a necessary condition for its indwelling. There is organized matter enough in the first cell that comes into existence after the fusion of the germ-nuclei to hold this rational form, or soul, as perfectly as it needs to be held in this first stage of human life.
To inform the embryo any principle, whether it is the rational soul or a force derived from the parental organism, must have organs; and if organs are present, then the embryo is fit to receive the human soul, as the only objection to its presence is a supposed lack of organs. To use other principles when the human soul itself could be present would be a multiplicatio entium sine necessitate, which is a condition repugnant to the universal method of the Creator.
It has been said that the vital activity in the fertilized ovum does not proceed from the rational soul because, "in the first place, it results from the fusion of two vital activities, neither of which is rational; secondly, it results in the formation, by fission, and differentiation, of two distinct and separate living cells, each containing within itself a principle of vital activity. Now this principle of vital activity cannot be a rational soul, for each cell has its own principle of activity, and in man there is but one soul."