This essay examines Augustus DeMorgan's chapter on fallacy in his Formal Logic (1847) in order to show how DeMorgan's treatment represents an expansion and advance upon Aristotle. It is important that Aristotle clearly distinguishes among dialectical, didactic, demonstrative, and contentious types of argument, based upon the acceptability of premises and the aims of participants. Appropriating Aristotle's list of fallacies, DeMorgan discusses examples that reveal how the charge and countercharge of fallacy function in contentious argument, which is more widespread than Aristotle (...) imagined. DeMorgan's treatment of fallacy is in the spirit of Aristotle because of its focus on dialogue arguments, but it represents an advance because it expands the possible scenes of contention and shows how unshared premises and the will to win inform many argument situations. The emphasis on contention in natural-language argument puts DeMorgan in the company of his l9th century peers, Mill and Whately. (shrink)