applies to Schuppe’s supposed syllogisms from two particular
premises:—
| (1) | (2) |
| Some M is P. | Some M is P. |
| Some S is M. | Some M is S. |
| ∴Some S may be P. | ∴Some S may be P. |
The only difference between these and the previous examples (2) and (3) is that, while those break the rule against two negative premises, these break that against undistributed middle. Equally fallacious are two other attempts of Schuppe to produce syllogisms from invalid moods:—
| (1) 1st Fig. | (2) 2nd Fig. |
| All M is P. | P is M. |
| No S is M. | S is M. |
| ∴S may be P. | ∴S is partially identical with P. |
In the first the fallacy is the indifferent contingency of the conclusion caused by the non-sequitur from a negative premise to an affirmative conclusion; while the second is either a mere repetition of the premises if the conclusion means “S is like P in being M,” or, if it means “S is P,” a non-sequitur on account of the undistributed middle. It must not be thought that this trifling with logical rules has no effect. The last supposed syllogism, namely, that having two affirmative premises and entailing an undistributed middle in the second figure, is accepted by Wundt under the title “Inference by Comparison” (Vergleichungsschluss), and is supposed by him to be useful for abstraction and subsidiary to induction, and by Bosanquet to be useful for analogy. Wundt, for example, proposes the following premises:—
| Gold is a shining, fusible, ductile, simple body. |
| Metals are shining, fusible, ductile, simple bodies. |
But to say from these premises, “Gold and metal are similar in what is signified by the middle term,” is a mere repetition of the premises; to say, further, that “Gold may be a metal” is a non-sequitur, because, the middle being undistributed, the logical conclusion is the contingent “Gold may or may not be a metal,” which leaves the question quite open, and therefore there is no syllogism. Wundt, who is again followed by Bosanquet, also supposes another syllogism in the third figure, under the title of “Inference by Connexion” (Verbindungsschluss), to be useful for induction. He proposes, for example, the following premises:—
| Gold, silver, copper, lead, are fusible. |
| Gold, silver, copper, lead, are metals. |
Here there is no syllogistic fallacy in the premises; but the question is what syllogistic conclusion can be drawn, and there is only one which follows without an illicit process of the minor, namely, “Some metals are fusible.” The moment we stir a step further with Wundt in the direction of a more general conclusion (ein allgemeinerer Satz), we cannot infer from the premises the conclusion desired by Wundt, “Metals and fusible are connected”; nor can we infer “All metals are fusible,” nor “Metals are fusible,” nor “Metals may be fusible,” nor “All metals may be fusible,” nor any assertory conclusion, determinate or indeterminate, but the indifferent contingent, “All metals may or may not be fusible,” which leaves the question undecided, so that there is no syllogism. We do not mean that in Wundt’s supposed “inferences of relation by comparison and connexion” the premises are of no further use; but those of the first kind are of no syllogistic use in the second figure, and those of the second kind of no syllogistic use beyond particular conclusions in the third figure. What they really are in the inferences proposed by Wundt is not premises for syllogism, but data for induction parading as syllogism. We must pass the same sentence on Lotze’s attempt to extend the second figure of the syllogism for inductive purposes, thus:—
| S is M. |
| Q is M. |
| R is M. |
| ∴Every Σ, which is common to S, Q, R, is M. |
We could not have a more flagrant abuse of the rule Ne esto plus minusque in conclusione quam in praemissis. As we see from Lotze’s own defence, the conclusion cannot be drawn without another premise or premises to the effect that “S, Q, R, are Σ, and Σ is the one real subject of M.” But how is all this to be got into the second figure? Again, Wundt and B. Erdmann propose new moods of syllogism with convertible premises, containing definitions and equations. Wundt’s Logic has the following forms:—
| (1) 1st Fig. | (2) 2nd Fig. | (3) 3rd Fig. |
| Only M is P. | x = y. | y = x. |
| No S is M. | z = y. | y = z. |
| ∴No S is P. | ∴x = z. | ∴x = z. |
Now, there is no doubt that, especially in mathematical equations, universal conclusions are obtainable from convertible premises expressed in these ways. But the question is how the premises must be thought, and they must be thought in the converse way to produce a logical conclusion. Thus, we must think in (1) “All P is M” to avoid illicit process of the major, in (2) “All y is z” to avoid undistributed middle, in (3) “All x is y ” to avoid illicit process of the minor. Indeed, it is the very essence of a convertible judgment to think it in both orders, and especially to think it in the order necessary to an inference from it. Accordingly, however expressed, the syllogisms quoted above are, as thought, ordinary syllogisms, (1) being Camestres in the second figure, (2) and (3) Barbara in the first figure. Aristotle, indeed, was as well aware as German logicians of the force of convertible premises; but he was also aware that they require no special syllogisms, and made it a point that, in a syllogism from a definition, the definition is the middle, and the definitum the major in a convertible major premise of Barbara in the first figure, e.g.:—
| The interposition of an opaque body is (essentially) deprivation of light. |
| The moon suffers the interposition of the opaque earth. |
| ∴The moon suffers deprivation of light. |
It is the same with all the recent attempts to extend the syllogism beyond its rules, which are not liable to exceptions, because they follow from the nature of syllogistic inference from universal to particular. To give the name of syllogism to inferences which infringe the general rules against undistributed middle, illicit process, two negative premises, non-sequitur from negative to affirmative, and the introduction of what is not in the premises into the conclusion, and which consequently infringe the special rules against affirmative conclusions in the second figure, and against universal conclusions in the third figure, is to open the door to fallacy, and at best to confuse the syllogism with other kinds of inference, without enabling us to understand any one kind.
3. Analytic and Synthetic Deduction.—Alexander the Commentator defined synthesis as a progress from principles to consequences, analysis as a regress from consequences to principles; and Latin logicians preserved the same distinction between the progressus a principiis ad principiata, and the regressus a principiatis ad principia. No distinction is more vital in the logic of inference in general and of scientific inference in particular; and yet none has been so little understood, because, though analysis is the more usual order of discovery, synthesis is that of instruction, and therefore, by becoming more familiar, tends to replace and obscure the previous analysis. The distinction, however, did not escape Aristotle, who saw that a progressive syllogism can be reversed thus:—
| 1. Progression. | 2. Regression. | |
| (1) | (2) | |
| All M is P. | All P is M. | All S is P. |
| All S is M. | All S is P. | All M is S. |
| ∴All S is P. | ∴All S is M. | ∴All M is P. |
Proceeding from one order to the other, by converting one of the premises, and substituting the conclusion as premise for the other premise, so as to deduce the latter as conclusion, is what he calls circular inference; and he remarked that the process is fallacious unless it contains propositions which are convertible, as in mathematical equations. Further, he perceived that the difference between the progressive and regressive orders extends from mathematics to physics, and that there are two kinds of syllogism: one progressing a priori from real ground
