One of the best known sources of the key phrase is the First Aphorism of Hippocrates (460-370 BCE), the famous physician/philosopher also known for the enduring oath that bares his name. Here's the aphorism in the original Greek:
Ὁ βίος βραχύς, ἡ δὲ τὲχνη μακρὴ, ὁ δὲ καιρὸς ὀξὺς, ἡ δὲ πεῖρα σφαλερὴ, ἡ δὲ κρίσις χαλεπή. Δεῖ δὲ οὐ μὸνον ἑωυτὸν παρέχειν τὰ δέοντα ποιεῦντα, ἀλλὰ καὶ τὸν νοσὲοντα, καὶ τοὺς παρεὸντας, καὶ τὰ ἔξωθεν
The passage opens with a phrase that is typically rendered in Latin (the scholarly language of the Middle Ages) as Vita Breva; Ars Longa, and from that comes the English, Life is short; Art is long, which has lost some of the character from the original by the time it reaches the English. The most common reading of the Hippocrates' aphorism alludes to the idea that the life of a physician is a rather short span of time to develop a thing so difficult as a medical practice. Hippocrates's literature on the whole can in some ways be cited as a source of the term and concept of the practice of medicine as an art. The meaning of art in this context includes the concept of science as well, which is the subject of this thread.
Use of the Greek τὲχνη (techne) in philosophy shares meaning with ἐπιστήμη (episteme), which is usually rendered knowledge or science in English. While both terms refer to knowledge, episteme is used to mean knowledge in the abstract, or knowing as an end in itself, whereas techne is used when discussing knowledge with a specific application, which sheds some light on other English words, like technique. For that reason, techne is often rendered as craft in the English, which can lead to a subtle misunderstanding of the term as simply knowing how to.
A useful illustration of the true meaning of the term, techne, is to consider how a maker of musical instruments will study the nature of sound and harmony in service of producing instruments that perform as intended, and how a musical performer will study some of the same theories of sound and harmony in service of rendering pleasing melody, and the studies musical composers pursue in service of rending pleasing combinations of melody, harmony, and rhythm. In that way, the term translates to the science and art, of this or that.
There's where Crowley's application of the phrase, science and art, is most apt, which bears directly on the Theory and Practice literary form he chose for his summary tome on magical arts as a practice. Of course, Crowley didn't originate that English phrase or the literary form. He was rather following well-worn tracks across the academic grounds of his own day.
For my money, knowing the etymology of the phrase offers some insights that bear directly on the contemporary practice of magic, particularly the oft overlooked value of theory in the face of so much internet-driven eclecticism. Long-term readers here will no doubt recall the dim view of eclecticism I tend to rehearse when discussing the cognitive conflicts that come into play from randomly selecting symbols for magical use that were derived from disparate cosmologies, for example. While many have decided that theories in magic are unnecessary, I continue to advance the idea that working from a theory, even a poor one, can have advantages over simple-minded eclecticism based primarily on the flotsam of the internet.
Furthermore, Hippocrates' entire First Aphorism offers added value to the contemporary seeker, when rendered this way:
Life is short, the practice long; opportunity is fleeting, experience delusive, judgement difficult.
Taken in the context of a medical or a magic practice, we are reminded that: 1) it's a life-long endeavor and we should not expect to perfect our craft in so short a time (beliefs in being an old soul notwithstanding); 2) learning events are inherently ephemeral and easy to miss if we're not dogged in our attention, 3) things are not always as they seem and we can expect to be misled by our own experiences at least some of the time, and 4) choosing a course of action is ever a challenge. The implication of the last point, for me personally, is that decision-making in a magic practice should never be done by rote or as a matter of course; rather, each magical act should be the result of a fresh deliberation on the very uniqueness of the situation.
Comments and personal insights are invited and welcome.