In Plato’s Phaedo, Socrates offers an argument for the soul’s immortality, dubbed the ‘Affinity Argument’ (78c-80b). Socrates argues that since the soul shares its essential properties with the invisible and the divine, it necessarily shares the further property of immortality. There was already debate among the Neoplatonists about the correct interpretation of the argument (Gertz 2011). The early history of the reception of the Affinity Argument, however, is often overlooked. Aristotle put forth a version (fr. 61 Rose), and the Peripatetics Strato (fr. 80 Sharples) and Boethus of Sidon (ap Eus. PE 184.108.40.206) objected to it. In this paper, I show that Cicero, too, plays a part in the history of the reception of the Affinity Argument.
Socrates in the Phaedo likens the soul to both the immaterial Forms (78c1-79e5) and god (79e8-80a5). I argue that Hellenistic philosophers focused on the latter, claiming that the intellective and motive powers of the soul are evidence for its divinity (Cic. de Sen. 77-8; de Rep. 6.26, [Josserand 1935]; cf. Ps-Plato, Axiochus 370b-c). Even Boethus rejects the Affinity Argument on the grounds that the similarity between soul and god is at once obvious to all, yet logically insufficient to prove immortality.
In Tusculan Disputations 1, Cicero adapts and strengthens the Affinity Argument. Cicero moves beyond the Hellenistic interpretation by emphasizing that knowledge of the soul is attainable through rational self-knowledge. Cicero does not merely assert the divine powers of the soul, but claims that the divinity—and thereby immortality—of the soul is universally accessible through introspection (cf. Brittain 2011). Cicero supports this by incorporating additional Platonic arguments from Alcibiades 1 (129a-32c ~ TD 1.52), Phaedrus (245c-46d ~ 1.53-5), and Timaeus (39a ~ 1.63)
I argue that Cicero’s Affinity Argument is philosophically robust and is formulated to respond specifically to the logical objection leveled against Plato’s original. Uniquely, Cicero defends not only the ontological similarities between the soul and the divine, but also their epistemic similarities. Knowledge of both the soul and god requires the same mode of inquiry (1.70: ut deum noris…sic animum tibi tuum notum esse; cf. 50-1, 66). While the essence (natura) of the divine or the soul may not be knowable, we can discern their powers (vis) and what they are like (qualis). This natural capacity for self-knowledge allows for rational inquiry into the soul. This epistemic affinity between soul and divine, I argue, allows Cicero to defend the logic of the argument against its detractors. The limited probative force of the Affinity Argument is not a flaw, but rather constitutive of any inquiry into either soul or god.
This discussion has two upshots. First it shows that Cicero was capable of formulating unique, unparalleled arguments in a Platonic vein. Second, as all good instances of reception should, it opens new avenues to explore the Platonic original. Following Cicero’s lead, we might try to read the Phaedo as Socrates’ final attempt to fulfil the Delphic maxim, gnothi seauton.
Philosophy in a Roman Context