ARISTOTLE'S PRIOR AND POSTERIOR ANALYTICS

THE PURE OR ASSERTORIC SYLLOGISM 33most nearly approaches perfection with regard to the valid moodsof the first figure; in dealing with them he simply claims that it isself-evident that any two premisses of the form All B is A, AllC is B, or NoB is A, All C is B, or All B is A, Some C is B, or NoH is A, Some C is B, warrant a certain conclusion in each case.But in his treatment of the invalid moods he does not point outthe formal error involved in drawing a conclusion, e.g. that ofreasoning from knowledge about part of a class to a conclusionabout the whole. He relies instead on empirical knowledge (orsupposed knowledge) to show that, major and middle term beingrelated in a certain way, and middle and minor term being relatedin a certain way, sometimes the major is in fact true of the minorand sometimes it is not. He thus shows that certain forms ofpremiss cannot warrant a conclusion, but he does not show whythey cannot do so.With regard to the other two figures, his chief defect is that henever formulates for them (as modem logicians have done) distinctprinciples of inference just as self-evident as the dictum deomni et nullo is for the first figure, but treats them throughoutoralmost throughout-as validated only by means of the firstfigure. In fact the only points at which he escapes from thetyranny of the first figure are those at which he uses ;K8EUt~ toshow the validity of certain moods. We have seen that his concentrationon the first figure follows from the lead given by Plato.But it would be a mistake to treat it as a historical accident.We must remember that Aristotle undertook the study of syllogismas a stage on the way to the study of scientific method. Nowscience is for him the knowledge of why things are as they are.And the plain fact is that only the first figure can exhibit this.Take the second figure. If we know that nothing having a certainfundamental nature has a certain property, and that a certainthing has this property, we can infer that it has not that fundamentalnature. But it is not because it has that property that ithas not that fundamental nature, but the other way about. Thepremisses supply a ratio cognoscendi, but not the ratio essendi, ofthe conclusion. Or take the third figure. If we know that allthings having a certain fundamental nature have a certain propertyand also a certain other property, we can certainly inferthat some things having the second property also have the first;but the fact that certain things have each of two properties is notthe reason why the properties are compatible; again we have only.985 D