In its own "proper region" no body is heavy or light; as we should say any fluid loses its weight when immersed in itself. When a portion of an element is out of its own region and surrounded by the great cosmic aggregate of another element, either of two cases may occur. The body which is "out of its element" may be below its proper place, in which case it is "light" and tends to move perpendicularly upwards to its place, or it may be above its proper place, and then it is "heavy" and tends to move perpendicularly "down" until it reaches its place. It was this supposed real distinction between motion "up" and motion "down" which made it so hard for the contemporaries of Galileo to understand that an inflated bladder rises for the same reason that a stone sinks.
Biology.--Of Aristotle's biology reasons of space forbid us to say much here. But a remark or two may be made about his theory of reproduction, since it is constantly referred to in much modern literature and has also played its part in theology. An interesting point is the distinction between "perfect" and "imperfect" animals. "Perfect" animals are those which can only be reproduced sexually. Aristotle held, however, that there are some creatures, even among vertebrates, which may be produced by the vivifying effect of solar heat on decomposing matter, without any parents at all. Thus malobservation of the facts of putrefaction led to the belief that flies and worms are engendered by heat from decaying bodies, and it was even thought that frogs and mice are produced in the same way from river-slime. In this process, the so-called "aequivocal generation," solar heat was conceived as the operative efficient cause which leads to the realisation of an organic "form" in the decaying matter.
In sexual reproduction Aristotle regards the male parent as the agent or efficient cause which contributes the element of form and organisation to the offspring. The female parent supplies only the raw material of the new creature, but she supplies the whole of this. No material is supplied by the male parent to the body of the offspring, a theory which St. Thomas found useful in defending the dogma of the Virgin Birth.
Psychology.--Since the mind grows and develops, it comes under the class of things which have a "source of motion internal to themselves," and psychology is therefore, for Aristotle, a branch of Physics. To understand his treatment of psychological questions we need bear two things in mind. (1) Psyche or "soul" means in Greek more than "consciousness" does to us. Consciousness is a relatively late and highly developed manifestation of the principle which the Greeks call "soul." That principle shows itself not merely in consciousness but in the whole process of nutrition and growth and the adaptation of motor response to an external situation. Thus consciousness is a more secondary feature of the "soul" in Greek philosophy than in most modern thought, which has never ceased to be affected by Descartes' selection of "thought" as the special characteristic of psychical life. In common language the word psyche is constantly used where we should say "life" rather than "soul," and in Greek philosophy a work "on the Psyche" means what we should call one on "the principle of life."
(2) It is a consequence of this way of thinking of the "soul" that the process of bodily and mental development is regarded by Aristotle as one single continuous process. The growth of a man's intellect and character by which he becomes a thinker and a citizen is a continuation of the process by which his body is conceived and born and passes into physical manhood. This comes out in the words of the definition of the soul. "The soul is the first entelechy (or actual realisation) of a natural organic body." What this means is that the soul stands to the living body as all form realised in matter does to the matter of which it is the form, or that the soul is the "form" of the body. What the "organic body" is to the embryo out of which it has grown, that soul is to the body itself. As the embryo grows into the actual living body, so the living body grows into a body exhibiting the actual directing presence of mind. Aristotle illustrates the relation by the remark that if the whole body was one vast eye, seeing would be its soul. As the eye is a tool for seeing with, but a living tool which is part of ourselves, so the body is a like tool or instrument for living with. Hence we may say of the soul that it is the "end" of the body, the activity to which the body is instrumental, as seeing is the "end" to which the eye is instrumental. But we must note that the soul is called only the "first" or initial "entelechy" of the body. The reason is that the mere presence of the soul does not guarantee the full living of the life to which our body is but the instrument. If we are to live in the fullest sense of the word, we must not merely "have" a soul; we "have" it even in sleep, in ignorance, in folly. The soul itself needs further to be educated and trained in intelligence and character, and to exercise its intelligence and character efficiently on the problems of thought and life. The mere "presence" of soul is only a first step in the progress towards fullness of life. This is why Aristotle calls the soul the first entelechy of the living body. The full and final entelechy is the life of intelligence and character actively functioning.
From this conception of the soul's relation to the body we see that Aristotle's "doctrine of body and mind" does not readily fall into line with any of the typical theories of our time. He neither thinks of the soul as a thing acting on the body and acted on by it, nor yet as a series of "states of mind" concomitant with certain "states of body." From his point of view to ask whether soul and body interact, or whether they exhibit "parallelism," would be much the same thing as to ask whether life interacts with the body, or whether there is a "parallelism" between vital processes and bodily processes. We must not ask at all how the body and soul are united. They are one thing, as the matter and the form of a copper globe are one. Thus they are in actual fact inseparable. The soul is the soul of its body and the body the body of its soul. We can only distinguish them by logical analysis, as we can distinguish the copper from the sphericity in the copper globe.
Grades of Psychical Life.--If we consider the order of development, we find that some vital activities make their appearance earlier than others, and that it is a universal law that the more highly developed activities always have the less highly developed as their basis and precondition, though the less highly developed may exist apart from the more highly developed. So we may arrange vital activities in general in an ontogenetic order, the order in which they make their appearance in the individual's development. Aristotle reckons three such stages, the "nutritive," the "sensitive," and the "intelligent." The lowest form in which life shows itself at all, the level of minimum distinction between the living and the lifeless, is the power to take in nutriment, assimilate it, and grow. In vegetables the development is arrested at this point. With the animals we reach the next highest level, that of "sensitive" life. For all animals have at least the sense of touch. Thus they all show sense-perception, and it is a consequence of this that they exhibit "appetition," the simplest form of conation, and the rudiments of feeling and "temper." For what has sensations can also feel pleasure and pain, and what can feel pleasure and pain can desire, since desire is only appetition of what is pleasant. Thus in the animals we have the beginnings of cognition, conation, and affective and emotional life in general. And Aristotle adds that locomotion makes its appearance at this level; animals do not, like plants, have to trust to their supply of nutriment coming to them; they can go to it.
The third level, that of "intelligence," i.e. the power to compare, calculate, and reflect, and to order one's life by conscious rule, is exhibited by man. What distinguishes life at this level from mere "sensitive" life is, on the intellectual side, the ability to cognise universal truths, on the conative, the power to live by rule instead of being swayed by momentary "appetition." The former gives us the possibility of science, the latter of moral excellence.[#]
[#] Cf. Dante's "Fatti non foste a viver como bruti,
Ma per seguir virtute e conosoenza."