 Most prominent theories of analogy, philosophical and computational, are based on overall similarity between source and target domains—defined in terms of some favoured subset of Hesse’s horizontal relations (see  §2.2).  Aristotle and Mill, whose approach is echoed in textbook discussions, suggest counting similarities. Hesse’s theory  (§3.3)  favours “pre-theoretic” correspondences. The structure-mapping theory and its successors  (§3.4)  look to systematicity, i.e., to correspondences involving complex, high-level networks of relations. In each of these approaches, the problem is twofold: overall similarity is not a reliable guide to plausibility, and it fails to explain the plausibility of any analogical argument.