It is a commonplace to say that abstraction has its degrees, as number its powers. Yet it is not sufficient to enunciate this truism; the degrees must be fixed by clear, objective signs, and these must not be arbitrary. Thus we shall obtain precise knowledge of the various stages in this ascending evolution, and stand in less danger of confounding abstractions highly distinct by nature. Moreover, we avoid certain equivocal questions and discussions that are based entirely upon the very extended sense of the terms to _abstract_ and to _generalise_. Accordingly we have sought to establish three main periods in the progressive development of these operations: (1) inferior abstraction, prior to the appearance of speech, independent of words (though not of all signs); (2) intermediate abstraction, accompanied by words, which though at first accessory, increase in importance little by little; (3) superior abstraction, where words alone exist in consciousness, and correspond to a complete substitution.[1] These three periods again include subdivisions, transitional forms which we shall endeavor to determine. This is a study of pure psychology, from which we have rigorously to eliminate all that relates to logic, to the theory of knowledge, to first principles of philosophy. We are concerned with genesis, with embryology, with evolution only. We are thus thrown upon observation, upon the facts wherein mental processes are enunciated, and discovered. Our material, and principal sources of information, lie therefore: (1) for inferior abstracts, in the acts of animals, of children, of uneducated deaf-mutes; (2) for intermediate abstracts, in the development of languages, and the ethnographical documents of primitive or half-civilised peoples; (3) for superior abstracts, in the progressive constitution of scientific ideas and theories, and of classifications. This volume is a _résumé_ of lectures given at the Collège de France in 1895. It is the first of a forthcoming series, designed to include all departments of psychology: the unconscious, percepts, images, volition, movement, etc. TH. RIBOT. March, 1897. TABLE OF CONTENTS. PAGE CHAPTER I. THE LOWER FORMS OF ABSTRACTION.—ABSTRACTION PRIOR TO SPEECH.