Δηλίου κολυμβητοῦ

This [sc. proverbial phrase] was uttered in reference to a book of Heraclitus, because it was hard to understand, that it needed a Delian diver who would not be drowned in it. Some entitle [this book] Muses, others On Nature. Diodotos [calls it] "A helm unerring for the rule of life," others "a guide of conduct, the ordering of character, for one [and] all."¹ Or thus: "of a Delian diver", in reference to those who swim deep. For when Euripides gave Socrates a book by Heraclitus the Obscure, [Euripides] asked, "How does it seem?" and [Socrates] said, "What I have understood [is] excellent, and I suppose what I have not understood [is] too -- except that it needs a Delian diver not to drown in it."²
And [there is] a proverb: "a Delian diver", in reference to those who are very experienced at swimming.³