When in classical formal logic the notions of deduction, valid inference and logical consequence are defined, causal language plays no role. The founder of western logic, Aristotle, identified 'non-cause', or 'positing as cause what is not a cause', as a logical fallacy (NCF). I argue that a systematic re-examination of Aristotle's analysis of NCF, and the related language of logical causality, in the Sophistical Refutations, Topics, Analytics and Rhetoric, helps us to understand his conception of sigma upsilon lambda lambda omicron gamma iota sigma mu omicron zeta. ('syllogism'). It reveals that Aristotle's syllogismhood is non-monotonic, and thus narrower than classical validity as consisting in, or coextensive with, necessary truth-preservation. It also supports the conclusion that a syllogism is a form of relevant consequence: the premisses must be causes of the conclusion in the sense that they are not only sufficient but also necessary for the conclusion to follow. Close analysis of NCF also suggests that syllogistic relevance is not a semantic matter, but a syntactic one: all premisses must be used in the chain of syllogistic deduction leading to the conclusion. Aristotelian syllogistic cannot therefore be accommodated within the framework of classical consequence-as its first interpretations 'from a modern standpoint' in the first half of the twentieth century attempted to do.
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