Experiments upon =abstraction= may be made in the manner outlined on p. 250: a complex stimulus (say, a visual stimulus that shows differences of colour, of number, of arrangement) is exhibited for a brief time; the observer is asked to attend to some one aspect of it (say, colour); and then, his report given, is asked to state what he can of the other aspects (number and form). Two general results may be mentioned. It is found, as might perhaps have been expected, that _things which make the least appeal to attention are also the things most easily overlooked_. Colour and form, for instance, are more attractive than number; and when the observer is told to attend to colour or form, number may go entirely unnoticed; whereas, when he is told to attend to number,—a relatively difficult task,—he is still able to say something of colour and form. The result seems only natural; but you may not see at once that it throws scientific light on a matter of some practical importance. We all know from sad experience that when thought, our own or another’s, flows smoothly and easily, it is likely to be superficial; the very smoothness of the flow means that difficulties have been overlooked. The obverse of this fact is, now, that if we struggle with the knotty points of a subject, we get a grip upon the whole; the interesting and attractive things take care of themselves; their native appeal to the attention keeps them in mind. So the experiments upon abstraction point a moral, at the same time that they illustrate the nervous mechanism of thought itself. They show, secondly, that _the negative effect of abstraction varies in degree_; the aspects of stimulus from which we abstract may be wholly suppressed, so that no report at all can be made of them, or may be apprehended indefinitely, so that the report is general; thus, form may be correctly named, while the colours are reported merely as ‘different,’ or as ‘dark.’ Another significant result! for it means that _a concept is more easily touched off than a special name_; we may fail to identify colours as red or blue when we can still say that they are dark or different. The reason is that the concept, the general name, is applied far oftener than the special name; its associative tendencies are therefore both deeper seated and more numerous. We have a parallel case in the image of p. 266, which slowly loses its distinctive features and approaches a type; and we have others in the gradual decay of memory with old age: a grandfather may forget the names of his grandchildren, but he does not forget that they are ‘boys’ and ‘girls.’ [Illustration]