In the localization of functions Plato adopts the current doc trine that the liver, the heart and the brain are the seat of desire, courage and reason. Democritus was probably responsible for this, and he was the leader of the scientific school which culti vated anatomy and favoured a materialistic doctrine based on the theory of atoms. From Democritus comes the long-lived theory that sensation is produced by the action on the body of atomic forms (eidOla) which are given off by objects, travel through the air and enter the body through appropriate channels of the five sense-organs. Plato had very little interest in the detail of sense perception. He concentrates attention on the experience rather than its physical or physiological basis and is content to take sensation as the beginning of knowledge. He treats knowledge as a growing sense of meaning, a form of reaction rather than a reception of impressions. This idea of activity as the most funda mental element in experience is the peculiarly Platonic element in all the schools which are opposed to a pure empiricism. The whole development from sense to reason is an inner development, a gradual evolution of the power of the soul to interpret events as they are given in the field of sensation. With a shrewd insight that has been justified by modern tests of intelligence, Plato con sidered that the distinctive mark of intellectual capacity was the understanding of relations. The synoptic mind, capable of seeing all things in their relations and therefore concerned primarily with meaning, is the highest product of human development. In this connection Plato elaborated his famous scheme of education. It is the first great example of applied psychology and also pre sents the germs of a social psychology. For education is regarded by Plato as a process in which the powers of the soul develop through the interaction between the individual and the environ ment. The first stage is the formation of a disposition as a result of influences unconsciously assimilated. The social environment is like an atmosphere: it furnishes the subtle influences which the mind feeds upon, analogous to the healthy or unhealthy air which imperceptibly produces health or disease in the body. This uncon scious assimilation of influences is called imitation. It is not conscious mimicry but the tendency to form ideals of action under the influence of the patterns which are ever before the eyes of the young. The later stage of education is comprised in definite forms of instruction of which the psychological character is that they develop the power of abstract thought.
Plato wrote no separate treatise on psychology: the subject appears at different points in his work whenever the questions of knowledge or conduct are under consideration. Aristotle is there fore usually considered the first teacher of a scientific psychology in the western schools. This title is well deserved, for the De Anima in particular and the other minor works associated with it in subject-matter are the sources from which have been drawn most of the psychology taught in Europe down to the close of the eighteenth century.
The main points of Aristotle's theory may be stated as follows: The animal organism begins its independent life with an endow ment of heat and motion. The material body has a connate pneuma : something corresponding to what a modern writer would call nervous energy is distributed throughout the body and makes possible the first functions. Aristotle knew nothing about nerves : he accepted in general the medical view that life is dependent on the presence of air in the blood vessels. This organic air was called pneuma, a word which means breath or wind, equivalent to the Latin anima or spiritus. The animal spirits were accepted almost without question to the end of the eighteenth century. They fulfilled in a confused manner all the functions of the breath as basis of life, of the nerves as conductors of sensation and of the more refined substance which was the assumed agent of rational thought. In a sense, therefore, the pneuma is the material soul,
a phrase which is not self-contradictory before the time when the soul as immaterial is opposed to the body as material. But Aris totle does not identify the soul with the pneuma. With consider able subtlety he defines the soul as the entelechy of a natural body capable of life. The word entelechy means the final state of development, natural completeness : and Aristotle really defines the soul as the functions of an organism when all its parts are united and active. The importance of this definition lies in its complete rejection of any attempt to make the soul a thing or an entity. The definition indicates a class of functions to be studied, and Aristotle proceeds to deal with them. All functions come under the general head of motion, so we are now really studying a particular class of motions. The body is a material structure subject to impact from the outer world : the genesis of sensations is found in the action of outer forces or stimuli on the organs of sense. Aristotle paid careful attention to the characteristics of touch, taste, smell, hearing and vision. In general all sense affection is a form of touch: the differences are in the medium and the sense-organ. In the case of vision a motion is transmitted from the source through a particular medium called the diaphan ous and continued along the pneuma of the optic passage to the sensorium. The form of this explanation is perfect: but the medium would now be defined as luminiferous ether (if any) and the optic passage with its pneuma as optic nerve. Af ter the separate senses comes the problem of unity. This requires a corn mon sensorium, a meeting place for all the special senses and also the centre for the perception of the common qualities of all objects, such as shape and size. Aristotle considered that the heart was the seat of all psychic functions, an unfortunate ab erration from the earlier and Platonic view that the brain was the organ of the soul. Probably Aristotle was led to this view by the fact that very little was known about the brain, nothing about nerves, and most emphasis was laid on the blood as the seat of life and of the pneuma. The error seems to have had little effect on the general description of functions. Aristotle saw the futility of defining sensation as the reception of any material image of the object or even as an impression : he evaded the difficulty by saying that sensation receives the form of its object without the matter: in other words sensation is a significant change of the mind corresponding to the physical motion in the sense-organ. As the original motion dies away a variety of different phenomena occur. After-images are motions continued in the organ of vision and are the most obvious example of the way in which motions persist. Imagination is a "decaying sense" and can be described as analogous to after-images. But here we have really begun to move away from any positive and perceptible motion: the words become metaphorical. If we admit a certain permanence in sense affections we can suppose that some sensations persist and that this is the origin of memory. The word in Greek conveys the idea of remaining or abiding (Mneme) and that is nearly all that can be said about memory. The accumulation of memories makes what men call experience. This is passive reason : but the passive must not be confused with the active. In spite of the strong empiricism of his method Aristotle does not confuse the mind with its physical basis or its acquired contents. Reason as the act of the soul is underived : whether the soul is or is not immortal Aristotle refuses to say.