Plutarch De E apud Delphos 7

[7] When Theon ceased, Eustrophus the Athenian, I think it was, said to us in answer, Do you see how zealously Theon defends logic, all but arraying himself in the lion’s skin? Under such conditions, we who repose in the Theory of Numbers all affairs together, natures and principles of things divine and human alike, and make this theory far above all else our guide and authority in all that is beautiful and valuable, should not be likely to hold our peace, but to offer to the god the first-fruits of our beloved mathematics, believing, as we do, that, taken by itself, E is not unlike the other letters either in power or in form or as a spoken word, but that it has come to be held in honour as the symbol of a great and sovereign number, the pempad, from which the wise gave the name pempazein to counting which is done by fives. ¹

These words Eustrophus addressed to us not in jest, but for the reason that at this time I was devoting myself to mathematics with the greatest enthusiasm, although I was destined soon to pay all honour to the maxim Avoid extremes, when I had once become a member of the Academy. ²

[7] Theon having ended his speech, I think it was Eustrophus the Athenian who said to us: Do you not see how valiantly Theon vindicates logic, having, in a manner, got on the lion’s skin? So it is not right even for us—who comprehensively place all the affairs, nature, and principles of things both divine and human in number, and make it most especially the author and lord of honest and estimable things—to be at quiet, but we must willingly offer the first-fruits of our dear mathematics to the God; since we think that this letter E does not of itself differ from the other letters either in power, figure, or expression, but that it has been preferred as being the sign of that great number which has an influence over all things, called the Quinary (or Pemptas), from which the Sages have expressed the art of numbering by the verb πεμπτάζειν (signifying to account by fives). Now Eustrophus spake these things to us, not in jest, but because I did at that time studiously apply myself to the mathematics, and perhaps also in every thing to honor that saying, Nothing too much, as having been conversant in the Academy.