The student will find that he will tend to acquire the habit of clear logical comparison and judgment, if he will memorize and apply in his thinking the following excellent Primary Rules of Thought, stated by Jevons:
"I. Law of Identity: The same quality or thing is always the same quality or thing, no matter how different the conditions in which it occurs.
"II. Law of Contradiction: Nothing can at the same time and place both be and not be.
"III. Law of Excluded Middle: Everything must either be, or not be; there is no other alternative or middle course."
Jevons says of these laws: "Students are seldom able to see at first their full meaning and importance. All arguments may be explained when these self-evident laws are granted; and it is not too much to say that the whole of logic will be plain to those who will constantly use these laws as their key."
CHAPTER XII.
DERIVED JUDGMENTS
As we have seen, a Judgment is obtained by comparing two objects of thought according to their agreement or difference. The next higher step, that of logical Reasoning, consists of the comparing of two ideas through their relation to a third. This form of reasoning is called mediate, because it is effected through the medium of the third idea. There is, however, a certain process of Understanding which comes in between this mediate reasoning on the one hand, and the formation of a plain judgment on the other. Some authorities treat it as a form of reasoning, calling it Immediate Reasoning or Immediate Inference, while others treat it as a higher form of Judgment, calling it Derived Judgment. We shall follow the latter classification, as best adapted for the particular purposes of this book.
The fundamental principle of Derived Judgment is that ordinary Judgments are often so related to each other that one Judgment may be derived directly and immediately from another. The two particular forms of the general method of Derived Judgment are known as those of (1) Opposition; and (2) Conversion; respectively.