First as to the mental states involved. The fact that the thinking deals with laws and principles necessitates the presence, in the thinking process, of constructive verbal or symbolic imagery, logical relationships, logical concepts, and explicit judgments. This does not at all exclude other types of these mental states and entirely different mental states. The kind of analysis involved simply necessitates the presence of these types, whatever others may be present. Constructive symbolic imagery has already been discussed. Logical relationships are those that are independent of accidental conditions, are not dependent on mere contiguity in time and space, but are inherent in the association involved. Such relationships are those of likeness and difference, cause and effect, subject and object, equality, concession, and the like. Logical concepts are those which are the result of thinking, whose definite meaning has been brought clearly into consciousness so that a definition could be framed. A child has some notion of the meaning of tree, or man, or chemist, and therefore possesses a concept of some kind, but the exact meaning, the particular qualities necessary, are usually lacking, and so it could not be called a logical concept. Explicit judgments are those which contain within themselves the reasons for the inference. They, too, are the result of thinking. One may say that "cheating is wrong," or that "water will not rise above its source level," or that "cleanliness is necessary to health," or that "this is a Rembrandt"--as a matter of experience, habit, but without any reflection and with no reasons for such judgment. If, on the other hand, the problems to which these judgments are answers had been a matter of thinking, the reasons or the ground for such judgment would have become conscious and the judgment then become explicit. It must be evident that in any problems dealing with laws and principles the mental states involved must be definite, clear cut, logically sound, and their implications thoroughly appreciated and understood.