§ 752. The special characteristic of the partly conjunctive syllogism lies in the transition from hypothesis to fact. We might lay down as the appropriate axiom of this form of argument, that 'What is true in the abstract is true--in the concrete,' or 'What is true in theory is also true in fact,' a proposition which is apt to be neglected or denied. But this does not vitally distinguish it from the ordinary syllogism. For though in the latter we think rather of the transition from a general truth to a particular application of it, yet at bottom a general truth is nothing but a hypothesis resting upon a slender basis of observed fact. The proposition 'A is B' may be expressed in the form 'If A is, B is.' To say that 'All men are mortal' may be interpreted to mean that 'If we find in any subject the attributes of humanity, the attributes of mortality are sure to accompany them.' CHAPTER XXII. _Of the Partly Conjunctive Syllogism regarded as an Immediate Inference_. § 753. It is the assertion of fact in the minor premiss, where we have the application of an abstract principle to a concrete instance, which alone entitles the partly conjunctive syllogism to be regarded as a syllogism at all. Apart from this the forms of semi-conjunctive reasoning run at once into the moulds of immediate inference. § 754. The constructive mood will then be read in this way-- If A is B, C is D, .'. A being B, C is D. reducing itself to an instance of immediate inference by subaltern opposition-- Every case of A being B, is a case of C being D. .'. Some particular case of A being B is a case of C being D. § 755. Again, the destructive conjunctive will read as follows-- If A is B, C is D, .'. C not being D, A is not B. which is equivalent to All cases of A being B are cases of C being D. .'. Whatever is not a case of C being D is not a case of A being B. .'. Some particular case of C not being D is not a case of A being B. But what is this but an immediate inference by contraposition, coming under the formula All A is B, .'. All not-B is not-A, and followed by Subalternation? § 756. The fallacy of affirming the consequent becomes by this mode of treatment an instance of the vice of immediate inference known as the simple conversion of an A proposition. 'If A is B, C is D' is not convertible with 'If C is D, A is B' any more than 'All A is B' is convertible with 'All B is A.' § 757. We may however argue in this way If A is B, C is D, C is D, .'. A may be B, which is equivalent to saying, When A is B, C is always D, .'. When C is D, A is sometimes B, and falls under the legitimate form of conversion of A per accidens-- All cases of A being B are cases of C being D. .'. Some cases of C being D are cases of A being B. § 758. The fallacy of denying the antecedent assumes the following form-- If A is B, C is D, .'. If A is not B, C is not D, equivalent to--