Plato etymologizes erōs at Cratylus 420a9–b4 through a phonetic-semantic analysis grounded in the idea of influx. Sexual desire is characterized as a flow that eisrhei exōthen (“flows in from outside”) through the eyes. On this basis, Plato derives the noun from the verb esrhein (“to flow in”), positing an earlier form esros, whose sound and sense encode external influx. The transition from esros to erōs is then explained by an alleged phonological change from ou to ō. The etymology is thus not historical in the modern sense but a philosophically motivated reconstruction linking sound, meaning, and a flux-based conception of sexual desire.

This etymology is embedded in a triadic cluster of specific desire-terms—himeros (“yearning desire,” 419e3–420a4), pothos (“longing,” 420a4–8), and erōs (“sexual desire,” 420a9–b4)—whose sequencing articulates a nuanced phenomenology of desire rather than a set of unrelated lexical curiosities. Himeros is analyzed as a present-directed and immediate desire: it arises from a flowing impulse (rhoē) that actively draws and pulls the soul toward what is currently before it. Pothos, by contrast, is described privatively in relation to himeros: it is desire for the same object, but precisely in its absence, thus marking a temporal and spatial displacement that converts immediacy into lack. Erōs is then distinguished not by presence or absence but by origin: it is an externally induced desire (eisrhein exōthen), entering through perception (the eyes) and foreign to the soul’s own internal motion. The triplet thus differentiates desire along two axes—mode of presence (himeros / pothos) and mode of causation (erōs)—and exemplifies a broader strategy of using etymology to map fine-grained psychological and conceptual distinctions rather than to provide merely historical word origins. 

These etymologies are embedded within a tradition of Greek myth and cult in which himeros, pothos, and erōs are not merely abstract mental states but closely related divine or daimonic figures with differentiated functions. Literary and archaeological evidence suggests, however, that this differentiation was neither stable nor consistently articulated. Cornutus (Greek Theology 25, 47.11–48.4 Lang = 40.1–17 Torres) reports the tradition of multiple Erōtes accompanying Aphrodite, but treats “Eros”, “Himeros”, and “Pothos” as alternative names of a single divinity. Pausanias (Description of Greece I.43.6.1–6, 101.18–23 Rocha-Pereira) reports that the temple of Aphrodite at Megara housed statues of Eros, Himeros, and Pothos, but pointedly adds: “if indeed their deeds correspond to the difference in their names” (εἰ δὴ διάφορά ἐστι κατὰ ταὐτὸ τοῖς ὀνόμασι καὶ τὰ ἔργα σφίσι, I.43.6.6–7, 101.23–4 Rocha-Pereira), thereby indicating that the three figures are not as easily distinguished as their names might suggest. For the literary relationship between Eros, Himeros, and Pothos, see Spencer 1932, 121; Weiss 1998, 31–2; Calame 1999, 30–3; Stafford 2013, 175–83; for the archaeological evidence, see Greifenhagen 1957, 34–9; Stafford 2013, 183–208. 

The broader philosophical context of Plato’s etymology of erōs at Cratylus 420a9–b4 is indicated by its derivation from eisrhein (“to flow into”). This etymology belongs to a wider sequence in which Plato explains names through flux terms; thus himeros, too, is derived from rhoē / rheuma (“flow,” 419e3–420a4). This pervasive flow vocabulary reflects a provisional adoption of the Heraclitean doctrine, endorsed by the character Cratylus, according to which all beings are in constant flux and names naturally encode processes rather than stable essences. Within this framework, naturalistic semantics—the view that names are correct by nature (phusei) rather than by convention (thesei)—finds its strongest motivation: if reality itself is fundamentally processual, then names must mirror motion and change. Through these flow-based etymologies, Plato radicalizes and dialectically tests both Heraclitean flux and naturalistic semantics, only to expose their joint inadequacy as a foundation for stable meaning and cognition—the central critical aim of the Cratylus. (On the relationship between naturalistic semantics and flux theory in the Cratylus, cf. e.g. Sedley 2003; Ademollo 2011; Meißner 2023.)

Plato offers alternative etymologies of erōs in the Phaedrus (238c; 252b). Plato’s etymologies of erōs in the Cratylus and the Phaedrus differ both linguistically and philosophically. In the Cratylus (420a9–b4), erōs is derived from esrhein (“to flow in”), yielding an earlier form esros and situating desire within a broader Heraclitean framework of flux: eros is conceived as an external influx entering the soul, and the etymology forms part of a systematic experiment in naturalistic semantics. By contrast, at Phaedrus 238c, erōs is explained from rhōmē (“strength”), emphasizing the overpowering psychic force of sexual desire, while at 252b it is associated mythopoetically with pteron (“wing”), presenting erōs as the power that lifts and transforms the soul. Whereas the Cratylus treats etymology as a philosophical test of whether names encode the causal structure of a world in flux, the Phaedrus employs etymology rhetorically and poetically to articulate different phenomenological aspects of desire.