7. RELATIVE VALUE OF THE FOUR FIGURES.
The first figure.
The first figure is known as the perfect figure; because it is the only one which proves all of the four logical propositions. Recalling the moods of the first figure makes this evident:
| A | E | A | E |
| A | A | I | I |
| A | E | I | O |
It is likewise the more natural figure; because it is the only one which uses both the subject and predicate of the conclusion in the same relative places as they appear in the premises. Symbolizing the figure makes this apparent:
M — G
S — M
S — G
The first figure, being the only figure which proves a “universal affirmation” (A), is used most by the scientist; as the object of science is to establish universal affirmative truths.
The second figure.
As the second figure conditions negative conclusions only, it is called the figure of disproof, or the exclusivefigure. It is easy to see how negative conclusions may be used to narrow the inquiry down to one definite theory. For example, suppose it is desired to ascertain which boy of the five broke the window; by a series of deductions the teacher may be able to prove that the culprit is not A, not B, not C and not D; hence the guilty one must be E. This figure is virtually the one used in diagnosing most diseases.
The third figure.
The third figure admits of particular conclusions only, and in consequence is of little value to the scientist.Since, however, the easiest way to contradict a universal affirmative (A) or a universal negative (E), is to prove the truth, respectively, of a particular negative (O) and a particular affirmative (I), it follows that the third figure serves a purpose.
The fourth figure.
This figure is so nearly like the first that it is of little value; in fact, it may be changed to the first by simply interchanging the major and minor premises. Some authorities refuse to recognize the fourth figure.