Expressed in this form, the reasoning process, as already mentioned, is known as a syllogism. The whole syllogism is made up of three parts, major premise, minor premise, and conclusion. The three concepts involved in the syllogism are known as the major, the minor, and the middle term. In the above syllogism, _heavy_, the predicate of the major premise, is the major term; _flint_, the subject of the minor premise, is the minor term; and _stone_, to which the other two are related in the premises, is known as the middle term. Because of this previous comparison of the major and the minor terms with the middle term, deduction is sometimes said to be a process by which the mind discovers a relation between two concepts by comparing them each with a third concept. =Purpose of Deduction.=--It is to be noted, however, as pointed out in Chapter XV, that deductive reasoning takes place normally only when the mind is faced with a difficulty which demands solution. Take the case of the boy and his lost coin referred to in Chapter II. As he faces the problem, different methods of solution may present themselves. It may enter his mind, for instance, to tear up the grate, but this is rejected on account of possible damage to the brickwork. Finally he thinks of the tar and resorts to this method of recovery. In both of the above cases the boy based his conclusions upon known principles. As he considered the question of tearing up the grate, the thought came to his mind, "Lifting-a-grate is a-thing-which-may-cause-damage." As he considered the use of the tar, he had in mind the judgment, "Adhesion is a property of tar," and at once inferred that tar would solve his problem. In such practical cases, however, the mind seems to go directly from the problem in hand to a conclusion by means of a general principle. When a woman wishes to remove a stain, she at once says, "Gasoline will remove it." Here the mind, in arriving at its conclusion, seems to apply the principle, "Gasoline removes spots," directly to the particular problem. Thus the reasoning might seem to run as follows: Problem: What will remove this stain? Principle: Gasoline will remove stains. Conclusion: Gasoline will remove this stain. Here the middle term of the syllogism seems to disappear. It is to be noted, however, that our thought changes from the universal idea "stains," mentioned in the statement of the principle, to the particular idea "this stain" mentioned in the problem and in the conclusion. But this implies a middle term, which could be expressed thus: Gasoline will remove stains; This is a stain; .'. Gasoline will remove _this_. The syllogism is valuable, therefore, because it displays fully and clearly each element in the reasoning process, and thus assures the validity of the conclusion.