Abstraction (Lat. abs, from; trahere, to draw) is a process (or a faculty) by which the mind selects for consideration some one of the attributes of a thing to the exclusion of the rest. With some writers, including the Scholastics, the attributes selected for attention are said to be abstracted; with others, as Kant and Hamilton, the term is applied to the exclusion of the attributes which are ignored; the process, however, is the same in both cases. The simplest-seeming things are complex, i.e. they have various attributes; and the process of abstraction begins with sensation, as sight perceives certain qualities; taste, others; etc. From the dawn of intelligence the activity progresses rapidly, as all of our generalizations depend upon the abstraction from different objects of some phase, or phases, which they have in common. A further and most important step is taken when the mind reaches the stage where it can handle its abstractions, such as extension, motion, species, being, cause, as a basis for science and philosophy, in which, to a certain extent at least, the abstracted concepts are manipulated like the symbols in algebra, without immediate reference to the concrete. This process is not without its dangers of fallacy, but human knowledge would not progress far without it. It is, therefore, evident that methods of leading the mind from the concrete to the abstract, as well as the development of a power of handling abstract ideas, are matters of great importance in the science of education.
With this account of the place of abstraction in the process of knowledge, most philosophers—and all who base knowledge on experience—are in substantial agreement. But they differ widely concerning the nature and validity of abstract concepts themselves. A widely prevalent view, best represented by the Associationist school, is that general ideas are formed by the blending or fusing of individual impressions. The most eminent Scholastics, however, following Aristotle, ascribe to the mind in its higher aspect a power (called the Active Intellect) which abstracts from the representations of concrete things or qualities the typical, ideal, essential elements, leaving behind those that are material and particular. The concepts thus formed may be very limited in content, and they vary in number and definiteness with the knowledge of particulars; but the activity of the faculty is always spontaneous and immediate; it is never a process of blending the particular representations into a composite idea, much less a mere grouping of similar things or attributes under a common name. The concept thus obtained represents an element that is universally realized in all members of the class, but it is recognized formally as a universal only by means of further observation and comparison. The arguments for the existence of such a faculty are not drawn from a study of its actual operation, which eludes our powers of introspection, but from an analysis of its results. Its defenders rely mainly on the fact that we possess definite universal concepts, as of a triangle, which transcend the vague floating images that represent the fusion of our individual representations; and also on the element of universality and necessity in our judgments. It is in connection with this latter point that the question is of most importance, as systems of philosophy which reject this power of direct abstraction of the universal idea are naturally more or less sceptical about the objective validity of our universal judgments.