The cause of this lies in the fact that the problem is at the outset at least partly understood by the learner, which fact enables him to determine whether the ideas coming forward in consciousness are related in any way to this partially known topic. Thus in the example cited, the learner knows the problem sufficiently to realize that it is a question of grammatical function, and is able, therefore, to feel the value, or suitability, of any knowledge which may be applied to it, even before he is fully aware of its ultimate relation thereto. LAW OF PREPARATION =Control of Old Knowledge Necessary.=--But notwithstanding the direction given the apperceptive process through the aim, or problem, it is evident that if the pupil is to select from his former experiences the particular elements which bear upon the problem in hand, he must have a ready and intelligent control over such former knowledge. It is too evident, however, that pupils frequently do not possess sufficient control over the old knowledge which will bear upon a presented problem. In endeavouring, for example, to grasp the relation of the exterior angle to the two interior and opposite angles, the pupil may fail because he has not a clear knowledge of the equality of angles in connection with parallel lines. For this reason teachers will often find it necessary (before bringing old knowledge to bear upon a new problem) to review the old knowledge, or experience, to be used during the apperceptive process. Thus a lesson on the participle may begin with a review of the pupils' knowledge of verbs and adjectives, a lesson on the making of the colours orange and green for painting a pumpkin with its green stem may begin with a recognition of the standard colours, red, yellow, and blue, and the writing of a capital letter with a review of certain movements. =Preparation Recalls Interpreting Ideas.=--It must be noted that this review of former knowledge always implies, either that the pupil is likely to have forgotten at least partially this former knowledge, or that without such review he is not likely to recall and apply it readily when the new problem is placed before him. For this reason the teacher is usually warned that his lesson should always begin with a review of such of the pupil's old knowledge as is to be used in mastering the new experiences. VALUE OF PREPARATION