Nor does this relating activity of mind confine itself within the single lesson. As each lesson is organized, it will, if fully apprehended, be more or less directly related with former lessons in the same subject. In this way the student should discover a unity within the lessons of a single subject, such as arithmetic or grammar. In like manner, various groups of lessons organize themselves into larger divisions within the subject, in accordance with important relations which the pupil may read into their data. Thus, in grammar, one sequence of lessons is organized into a complete knowledge of sentences; another group, into a complete knowledge of inflection; a smaller group within the latter, into a complete knowledge of tense or mood. It is thus that the mind is able to construct its mass of knowledge into organized groups known as sciences, and the various smaller divisions into topics. CHAPTER XII APPLICATION OF KNOWLEDGE OR LAW OF EXPRESSION =Practical Significance of Knowledge.=--In our consideration of the fourth phase of the learning process, or the law of expression, it is necessary at the outset to recall what has already been noted regarding the correlation of knowledge and action. In this connection it was learned that knowledge arises naturally as man faces a difficulty, or problem, and that it finds significance and value in so far as it enables him to meet the practical and theoretical difficulties with which he may be confronted. In other words, man is primarily a doer, and knowledge is intended to guide the conduct of the individual along certain recognized lines. This being the case, while instruction aims to control the process by which the child is to acquire valuable social experience, or knowledge, it is equally important that it should promote skill by correlating that knowledge with expression, or should strive to influence action while forming character. To apperceive, for instance, the rules of government and agreement in grammar will have a very limited value if the student is not able to give expression to these in his own conversation. It becomes imperative, therefore, that as far as possible, expression should enter as a factor in the learning process.