Volume 10, 2018
We offer a formulation of a set of rules for definitions that is informed by modern logic. We aim to be as precise as possible in our formulation. The set of rules that we discuss derives from Aristotle’s treatise on the art of dialectic, Topics. The concern about rules for definitions can be traced back at least to Socrates, as represented in Plato’s early dialogues. Since we view our task as belonging to general philosophical methodology and as being central to it, we approach the rules for definitions from a general perspective and try to avoid adjudicating controversial issues in scientific methodology or contemporary theories of meaning. We discuss some philosophical difficulties as we proceed. First, we distinguish three components in a rule: a principle, a criterion and a motivation. Secondly, we discuss the logical form of definition sentences and the properties of the relation “…=df …”. Thirdly, we account for six classical rules, highlighting the components for each rule. The rules address issues about extensional equality, essential predication, circularity, negative definitions, synonymous expressions and metaphorical language. Our formulation makes it apparent that the principles of definition are either logical requirements or pragmatic rules.