Commensuration and Currency in Plato’s Phaedo

Abstract My aim in this paper is to show that Plato’s Phaedo makes an important contribution to the development of ideas about the commensuration in value of heterogeneous items that is needed for practical reasoning and rational choice. Because the passage I focus on, the so-called ‘right exchange’ passage at 69 a-c , has not usually been read this way, I motivate the reading by showing how it resolves some puzzles local to the Phaedo concerning the stark contrast Socrates develops between the virtues of philosophers and nonphilosophers, ordinary people who do not pursue wisdom single-mindedly, applying to the latter labels like ‘strange’, ‘unreasonable’, ‘illusory’ and ‘slavish’. Socrates says that one should, and suggests that philosophers do, use wisdom as their currency whereas nonphilosophers use pleasure (and perhaps also pains, fears, etc., or bodily conditions in general) as their currency (or currencies). Although it has been generally understood that the basis for this contrast lies in how the two groups evaluate, the details have remained murky. I argue that it’s due to the properties of pleasure (or bodily conditions in general) qua measure of value, in particular pleasure’s context-dependence, that nonphilosophers’ judgements about virtue, e.g. the courage and moderation of particular acts or people, are defective in the ways Socrates says they are.