The argument is rather weak. Even if we agree that water is always present in nourishment and there are no completely dry foods (a cracker may have some particles of water from the air or from the mouth), this may only mean that water is needed as a lubricant to the intake of food. In the context of discussing “cruder thinkers,” like Hippon, who thought that the soul is water, Aristotle modiﬁes the argument by saying that the choice of water “seems” to have been determined by the idea that “the seed of everything is moist” (De anima 405b2–3 = 31A4), which does not considerably strengthen the argument. A stronger argument is given by Theophrastus who says that the choice of water was caused by the observation that corpses dry up (ap. Simplicius, In Phys. 23.21–29 = A13). In these arguments, water is considered a source of life: indispensable to maintain life through nourishment, to begin life through the seed, so that life ends when water vanishes. In this way, water is not made a principle or a substrate of everything, but only a seat of life. If there were no water, a lifeless universe could still exist. One possible reason for the choice of water is Thales’ strong interest in nautical matters (building canals, explaining the periodical ﬂoods of the Nile, diverting the river Halys),12 but this would explain at best his interest in mathematics, astronomy, and the like, but not in cosmology and philosophy.13 Another reason is simply the fact that Thales lived close to the sea.14 But so did Anaxagoras and Anaximenes.
A more comprehensive reason is given by Heraclitus Homericus when he says that water is “easily formed into each different thing”: into slime and earth when compacted, into air when exhaled, and “the ﬁnest part is kindled from air into aether” (Quest. Hom. 22).15 However, the same case can be made about any other substance using the same reasoning; for example, aether is a substrate because when compacted, it becomes air, and the crudest part is compacted from air into water, and so on.
It seems that the primary reason for the choice of water as a substrate should not be sought in physiology or geography, but in religion. In that direction points an explanation that Thales was inspired by the cult of Poseidon who keeps earth in its place in the sea.16 But it seems that a less parochial view of religion is required to lead to the view of water being a cosmic principle.
Already some ancient authors stated that Thales took his ideas on water from Egypt.17 There are claims that Thales “practiced philosophy in Egypt” (Aetius 1.3.1; Proclus, In Eucl. 65 = A11), where he traveled and even was taught by priests (DL (Diogenes Laertius, Vitae philosophorum) 1.27 = A1). The claim of his
12 Benjamin Farrington, The Faith of Epicurus (New York: Basic Books, 1967), p. 39.
13 Josef Dörﬂer, ‘Die kosmogonischen Elemente in der Naturphilosophie des Thales’, Archiv für Geschichte der Philosophie 25 (1912), p. 313.
14 Michel Costantini, La Génération Thalès (Paris: Criterion, 1992), p. 92. Therefore, “rise and ebb, water that spreads softly on the beach, distant scintillations of the many sails, this is what nourished his meditation,” p. 85.
15 The same argument is used by J.C. Davies, ‘Mythological inﬂuences on the ﬁrst emergence of Greek scientiﬁc and philosophical thought’, Folklore 81 (1970), p. 28.
16 August B. Krische, Die theologischen Lehren der griechischen Denker (Göttingen: Dieterichsche Buchhandlung, 1840), pp. 35–36.
17 Plutarch, De Is. et Os. 34 = A11; Simplicius, In De caelo 522.14–18 = A14.