Philosophers and scientists of the Renaissance did not treat psychology, the philosophical study of the soul, as an independent discipline. Following the medieval tradition, they placed it within the broader context of natural philosophy, and they approached it, like the other sub-divisions of natural philosophy, through the works of Aristotle, notably De anima and the Parva naturalia. The term psychologia itself was coined–apparently by the German humanist Joannes Thomas Freigius in 1575 – to refer to the traditional complex of problems originating from these two works. Thus it is in relation to the Aristotelian tradition, and more specifically to the Aristotelian philosophy of nature, that the meaning and content of psychology in this period must be defined.

Aristotle and his followers defined the soul as the life principle of the individual body – that which differentiated living from non-living things. As such it was the source and formal cause of the specific functions and activities of animate beings, including plants and animals as well as men. Thus before the seventeenth century, when Descartes, Stahl and others moved to divorce the notions of life and soul, there was no clear division between psychology and what we now call biology. Although Renaissance writers emphasised problems of cognition, emotion and volition (the main subjects of De anima), the field also included a good deal of plant and animal physiology, based not only on the Parva naturalia but also on the ‘animal books' of Aristotle and to a lesser extent on the pseudo-Aristotelian De plantis.

As the scale of this volume attests, the period of the Renaissance was one of intense philosophical activity. It is only recently, however, that the extent of this activity has come to be recognised fully. Although eighteenth-century historians of philosophy such as Jakob Brucker saw in the Renaissance an important period of reorientation, their awareness did not in general carry over into nineteenth-century attempts at historical synthesis. Burckhardt's celebrated essay remained virtually silent about the contributions of philosophy to the civilisation of the Renaissance, while Anglo-Saxon traditions of scholarship generally treated the two centuries after the death of William of Ockham, if at all, merely as a backdrop to the heroic age of Francis Bacon and the ‘new philosophy’. A few Renaissance thinkers – Ficino, Bruno, Campanella – occasionally found a place in nineteenth-century histories, but even then the interpretation of their work tended to remain deficient in several respects. One problem was that the kind of historical research needed to make possible a comprehensive evaluation of Renaissance philosophy largely remained to be carried out. A further weakness derived from the fact that most nineteenth-century historians were more interested in tracing the roots of ‘modern’ thought than in considering the ebb and flow of philosophical teaching and speculation at different times. Even when Renaissance writers were discussed, they were generally treated as pawns in the philosophical battles of later centuries, not as thinkers of their own age and in their own right.

Aristotle's teaching on the intellective soul (De anima III.4–5) serves as the starting-point for Renaissance discussions and, therefore, predetermines the questions raised and the answers given. In the Averroist tradition, this was treated as the beginning of the entire third book. Chapter 4 attempts to define the activity of the intellective soul through analogy to sense-perception and by so doing introduces an interdependence between psychological and epistemological theories. In the fifth chapter, distinguishing between the possible and the agent intellect, Aristotle goes beyond the analogy with sense-perception and alludes to the active role of the soul in the process of knowing. This extremely condensed and enigmatic chapter has provoked many different interpretations, ranging from the outright denial of the agent intellect to the postulation of an agent sense as well, in order to maintain the analogy with sense-perception. For those commentators, however, who kept between these two extremes, III.5 provided both the chance and the need for metaphysical speculation on the ontological status of the intellective soul including its relation to the celestial intelligences and the question of its immortality. It was to this last question that particular attention had to be paid, since on the one hand, Aristotle is not explicit about it, and on the other, Christian doctrine required an affirmation. Thus, for the Middle Ages the question was not whether the human soul was immortal but rather how an immortal soul could fit into the ontological structure of the universe. Consequently, the metaphysical point of view gained prominence, until, in fourteenth-century nominalism, metaphysics lost ground and a new approach was possible from the perspectives of natural philosophy and epistemology.