The general decline of western culture brought to an end all progress in the natural sciences and philosophy. From the fifth to the twelfth century nothing important is achieved. The revival of learning in the twelfth century and its more complete restora tion in the thirteenth was due in large part to the rise and progress of the Arab empire. When the victorious progress of Islam in the seventh century made the Arab master of the Persian empire, the Greek culture which survived in Syria and elsewhere was eagerly absorbed by the conquerors. From Baghdad in the East it travelled to Spain in the West, and, with additions from Con stantinople and Sicily, it reappeared in the thirteenth century as the new learning of Christendom. So far as concerns any material that is important the psychology taught in the thirteenth century is simply the Greek psychology so far as it was recovered and understood. Aristotle and Augustine together account for all the available material.
Progress in psychology depends mainly on two kinds of inter est. One is the interest in the structure and functions of the body which provides new material in the fields of anatomy and physi ology. The other is the interest in the variety of human behaviour as it is observed in personal or social relations. During the period from the thirteenth to the sixteenth century there was a slow accumulation of efforts in both these directions and the result was seen in a great revival of activity which must be regarded as a subsidiary effect of the main progress in the sciences and in humanism. The work of Vesalius in human anatomy (1543) is the principal landmark in the study of the structure of the body : its ultimate significance was the tendency to abandon the view that a number of faculties were located in the body (such as the faculty of digestion or excretion) and to reduce all processes to the terms of physiological mechanism. By successive attacks on the idea of the soul as something residing in the body and dis tinct from its material organization, the writers of the Renaissance built up a new science of man which had for its main principle the idea of natural functions entirely distinct from the activity of a supernatural soul. This movement was part of the whole develop ment by which science becathe separated from theology.
The first part of the movement was quite unsystematic and was achieved as the expression of new values in the estimate of human conduct. The real beginnings are found in the work of such writers as Machiavelli who sweep away the formal descrip tions of conduct as uniformly rational and openly declare that man is ruled by elementary passions. This example was followed by many others. Whether they accepted the idea that man is by nature evil or preferred to describe human nature more gener ously, they all agreed that conduct depends on natural impulses, that desire is more important than reason, and behaviour expresses temperament. As temperament could be explained in physiological language and related to the humours, it is evident that this kind of theory ceased to have any connection with the traditional doc trine that man was made in the likeness of God. A powerful movement toward reform in education assisted the progress of these views. A good example of the relation between psychology and education at this period is the work of Juan Luis Vives, a Spaniard who travelled widely and produced a marked effect on English thought after his lectures in Oxford (1523-28). The inter est felt at that time in the study of personality was shown by the great popularity of a book called The Examination of Men's Wits, translated from the original Spanish work of Juan Huarte. A
flood of writings too numerous to mention followed the fashion of this age. Discussion of individual differences as due to age, sex, climate or other causes filled many volumes and anticipated the better known works of the late eighteenth century. The pro gramme carried out by later investigators was outlined by Francis Bacon in his Advancement of Learning (1605). Bacon appreciated fully the new attitude in psychology and advocated research along lines which have only been adequately treated in modern times. Among Bacon's suggestions are such topics as the use of aids to memory, the importance of connecting concepts with impressions on the senses, and the whole range of studies by which the founda tions of character can be discovered in the constitution of the body and the social environment. In this way Bacon directed attention to the kind of material which now belongs to social psy chology.
Thomas Hobbes (1588-1679) is often described as the father of modern psychology. The phrase is justified by the fact that Hobbes produced a concise summary of psychology which in cluded both the individual and the social aspects. Taking as his guiding principle the idea of motion, Hobbes explicitly rejected the scholastic phrases and developed a mechanistic scheme. Sen sation is defined as a mode of motion excited in the physiological organism : imagination is a continuation of this motion after the object ceases to act on the senses : dreams are imagery caused by external conditions, such as a sensation of cold, which produce internal motions equivalent to imagination : reason is the adding and subtracting of ideas derived from 'experience. The method used by Hobbes was derived from the physical sciences and par ticularly from the mechanics of Galileo. It was deductive and arbitrary in character but it influenced the whole trend of thought by showing the possibility of a psychology which conformed to the new ideas of scientific procedure. In general Hobbes drew upon Aristotle for much of his material and some of his most striking phrases owe their origin to his knowledge of the original Aristotelian language. In a famous passage of the Leviathan (Ch. 3) Hobbes explains the connection between ideas as due to the "trayne of imaginations" and gives an example of the way in which one idea may draw another after it. Hobbes deserves credit for the way in which he treats the will. As sensation is a move ment inward, due to the action of objects, so volition is a move ment outward and a kind of reaction which is primarily a form of vital movement. Hobbes thus begins the account of will from the basis of a natural striving or activity which was called at that time appetite. This elementary striving is connected with pleasure or pain, and is accordingly a movement toward or away from the exciting cause. As the creature develops, this activity is asso ciated with images, becomes modified by deliberation, and so finally appears as will. Will is therefore defined simply as "the last appetite." This again is really a version of Aristotle's doctrine in the Ethics, but it was restated in a style unsurpassed for viril ity and may be accepted as original on account of the new value which the ideas received from the form in which they were pre sented. The ultimate aim of Hobbes was to give an account of human conduct in social relations and to explain the necessity of political order to control natural passions. His views are those of a sociologist who requires to begin with a scientific description of the human species. Hobbes was essentially a behaviourist who lacked the necessary knowledge of biology and was compelled to employ the of a mechanistic age.