Why do we say "One Swallow Does Not Make a Summer"?

Meaning:

Don't draw a premature conclusion based on a single fact

Background:

his proverb appears in Aristotle's Nicomachean Ethics - the name given to Aristotle's best known work on ethics which is believed to be based on notes from his lectures at the Lyceum in Athens. Aristotle died in 322 BC.

"Now we take the human function to be a certain kind of life, and take this life to be the soul's activity and actions that express reason, and the function of a good man to be the good and noble performance of these. Each function is completed well when its completion expresses the proper virtue. Therefore, the human good turns out to be the soul's activity that expresses virtue. And if there are more virtues than one, the good will express the best and most complete virtue. Moreover, it will be in a complete life. For one swallow does not make a spring, nor does one day; nor, similarly, does one day or a short time make us blessed and happy."

(translated by Terence Irwin, Hackett Publishing Co. 1985)

Many fables have been written based on the proverb including one attributed to Aesop. However, considering that Aesop lived around 600 BC (or at least the legend that became Aesop appears to have originated around this time as there's some uncertainty whether Aesop actually existed), it would seem that the proverb dates back at least a few hundred years before Aristotle.

In Aesop's fable, a young man sees a swallow on a warm winter day. Thinking that winter is over, he sells off his woolen coat, and with the money he's made he goes to the bar and drinks. Unfortunately, in the days that follow the temperature drops and the young man, shivering in the cold, realizes that one swallow does not make a summer.

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