=Value of Scientific Knowledge.=--In the practice of any intelligent occupation or art, in so far as the practice attains to perfection, there are manifested in the processes certain scientific principles and methods to which the work of the one practising the art conforms. In the successful practice, for example, of the art of composition, there are manifested the principles of rhetoric; in that of housebuilding, the principles of architecture; and in that of government, the principles of civil polity. In practising any such art, moreover, the worker finds that a knowledge of these scientific principles and methods will guide him in the correct practice of the art,--a knowledge of the science of rhetoric assisting in the art of composition; of the science of architecture, in the art of housebuilding; and of the science of civil polity, in the art of government. =The Science of Education.=--If the practice of teaching is an intelligent art, there must, in like manner, be found in its processes certain principles and methods which may be set forth in systematic form as a science of education, and applied by the educator in the art of teaching. Assuming the existence of a science of education, it is further evident that the student-teacher should make himself acquainted with its leading principles, and likewise learn to apply these principles in his practice of the art of teaching. To this end, however, it becomes necessary at the outset to determine the limits of the subject-matter of the science. We shall, therefore, first consider the general nature and purpose of education so far as to decide the facts to be included in this science. CONDITIONS OF GROWTH AND DEVELOPMENT =A. Physical Growth.=--Although differing in their particular conception of the nature of education, all educators agree in setting the child as the central figure in the educative process. As an individual, the child, like other living organisms, develops through a process of inner changes which are largely conditioned by outside influences. In the case of animals and plants, physical growth, or development, is found to consist of changes caused in the main through the individual responding to external stimulation. Taking one of the simplest forms of animal life, for example, the amoeba, we find that when stimulated by any foreign matter not constituting its food, say a particle of sand, such an organism at once withdraws itself from the stimulating elements. On the other hand, if it comes in contact with suitable food, the amoeba not only flows toward it, but by assimilating it, at once begins to increase in size, or grow, until it finally divides, or reproduces, itself as shown in the following figures. Hence the amoeba as an organism is not only able to react appropriately toward different stimuli, but is also able to change itself, or develop, by its appropriate reactions upon such stimulations.