Aristotle's Psychology. On the Soul, or De anima

This book treats of the Problem of Psychology and the attempts which have been made to solve it; The Nature of Soul and its specific Forms; The Intellectual and Active Powers. "The acquisition of knowledge is always something high and honourable; but one form of knowledge is superior to another either in virtue of the self-contained simplicity of its truths or by the greater dignity and wondrousness of its contents: and on both these grounds the investigation of the soul might with justice claim a foremost place.
And, besides, the knowledge of it is thought to have important bearings on truth generally and especially on nature: for soul is as it were the prime factor in animal existence. The object of our enquiry is to observe and to discover both the historical development and the essential nature of the soul, and further to find out the phenomena occurring in connection with it - phenomena of which some are thought to be affections peculiar to the soul itself, others, while owing their existence to the soul, are thought to belong to the animal nature taken as a whole..."