§ 74. Special forethought is necessary lest the brain be too early over-strained, and lest, in consequence of such precocious and excessive action, the foundation for a morbid excitation of the whole nervous system be laid, which may easily lead to effeminate and voluptuous reveries, and to brooding over obscene representations. The excessive reading of novels, whose exciting pages delight in painting the love of the sexes for each other and its sensual phases, may lead to this, and then the mischief is done. SECOND DIVISION. INTELLECTUAL EDUCATION. § 80. _Mens sana in corpore sano_ is correct as a pedagogical maxim, but false in the judgment of individual cases; because it is possible, on the one hand, to have a healthy mind in an unhealthy body, and, on the other hand, an unhealthy mind in a healthy body. To strive after the harmony of soul and body is the material condition of all proper activity. The development of intelligence presupposes physical health. Here we are to speak of the science of the art of Teaching. This had its condition on the side of nature, as was before seen, in physical Education, but in the sphere of mind it is related to Psychology and Logic. It unites, in Teaching, considerations on Psychology as well as a Logical method. FIRST CHAPTER. _The Psychological Presupposition._ § 81. If we would have a sound condition of Philosophy, it must, in intellectual Education, refer to the conception of mind which has been unfolded in Psychology; and it must appear as a defect in scientific method if Psychology, or at least the conception of the theoretical mind, is treated again as within Pedagogics. We must take something for granted. Psychology, then, will be consulted no further than is requisite to place on a sure basis the pedagogical function which relates to it. § 82. The conception of _attention_ is the most important to Pedagogics of all those derived from Psychology. Mind is essentially self-activity. Nothing exists for it which it does not itself posit as its own. We hear it not seldom implied that something from outside conditions must make an impression on the mind, but this is an error. Mind lets nothing act upon it unless it has rendered itself receptive to it. Without this preparatory self-excitation the object does not really penetrate it, and it passes by the object unconsciously or indifferently. The horizon of perception changes for each person with his peculiarities and culture. Attention is the adjusting of the observer to the object in order to seize it in its unity and diversity. Relatively, the observer allows, for a moment, his relation to all other surroundings to cease, so that he may establish a relation with this one. Without this essentially spontaneous activity, nothing exists for the mind. All result in teaching and learning depends upon the clearness and strength with which distinctions are made, and the saying, _bene qui distinguit bene docit_, applies as well to the pupil.