ABSTRACT
The classical doctrine views all concepts as formed by abstracting the marks or characteristics common to all instances to which the concept applies. Any survey of the actual concepts of modern science will show that they are all predominantly concerned with relations, operations, or transformations rather than with classes or kinds of things and their qualities. That the relative extent of twilight zone and focal illumination varies with different concepts can be made evident if we barely juxtapose concepts like long distance, a day's journey, and thirty-five miles, or a good machine and one that is seventy per cent, efficient, or an unpopular magistrate and one that has a majority against him. The twilight zones between the classes of plants and animals, between vertebrates and invertebrates, need not disturb the biologist whose principles of diversion are based on concepts of wide and significant application.
