              In Aristotle’s logic, the basic ingredients of reasoning are given in terms of inclusion and exclusion relations, of the sort graphically captured many years later by the device of Venn diagrams. He begins with the notion of a patently correct sort of argument, one whose evident and unassailable acceptability induces Aristotle to refer to is as a ‘perfect deduction’ (APr. 24b22–25). Generally, a deduction (sullogismon), according to Aristotle, is a valid or acceptable argument. More exactly, a deduction is ‘an argument in which when certain things are laid down something else follows of necessity in virtue of their being so’ (APr. 24b18–20). His view of deductions is, then, akin to a notion of validity, though there are some minor differences.   For example, Aristotle maintains that irrelevant premises will ruin a deduction, whereas validity is indifferent to irrelevance or indeed to the addition of premises of any kind to an already valid argument. Moreover, Aristotle insists that deductions make progress, whereas every inference from p to p is trivially valid. Still, Aristotle’s general conception of deduction is sufficiently close to validity that we may pass into speaking in terms of valid structures when characterizing his syllogistic. In general, he contends that a deduction is the sort of argument whose structure guarantees its validity, irrespective of the truth or falsity of its premises. This holds intuitively for the following structure: