§ 105. (2) But the subject must be adapted to the consciousness of the pupil, and here the order of procedure and the exposition depend upon the stage which he has reached intellectually, for the special manner of the instruction must be conditioned by this. If he is in the stage of perception, we must use the illustrative method; if in the stage of conception, that of combination; and if in the stage of reflection that of demonstration. The first exhibits the object directly, or some representation of it; the second considers it according to the different possibilities which exist in it, and turns it around on all sides; the third questions the necessity of the connection in which it stands either with itself or with others. This is the natural order from the stand-point of the scientific intelligence: first, the object is presented to the perception; then combination presents its different phases; and, finally, the thinking activity circumscribes the restlessly moving reflection by the idea of necessity. Experiment in the method of combination is an excellent means for a discovery of relations, for a sharpening of the attention, for the arousing of a many-sided interest; but it is no true dialectic, though it be often denoted by that name. ——Illustration is especially necessary in the natural sciences and also in æsthetics, because in both of these departments the sensuous is an essential element of the matter dealt with. In this respect we have made great progress in charts and maps. Sydow's hand and wall maps and Berghaus's physical atlas are most excellent means of illustrative instruction; also Burmeister's zoölogical atlas.——