Isidore of Seville, Etymologies 1.1:
Instruction (disciplina) received its name from learning (discendo): for this reason, it can also be called knowledge (scientia), because to know (scire) is derived from to learn (discere), because no one knows something if they have not learned it. Alternatively, it is called disciplina because it is learned in its fullness (discitur plena). Art however is so called because it consists of strict (artis) precepts and rules. Others say that this word is derived from the Greeks, from arete, that is, from virtue, which they called knowledge.
Plato and Aristotle wanted to establish this difference between art and instruction, saying that art (ars) consists of those things which can come about in an alternative way; but instruction (disciplina) deals with those things which cannot be otherwise than they are. For when something is discussed in true disputations, it is instruction; but when something similar to the truth and depending on conjecture is under discussion, it has the name of art.
DE DISCIPLINA ET ARTE.  De disciplina et arte. Disciplina a discendo nomen accepit: unde et scientia dici potest. Nam scire dictum a discere, quia nemo nostrum scit, nisi qui discit. Aliter dicta disciplina, quia discitur plena.  Ars vero dicta est, quod artis praeceptis regulisque consistat. Alii dicunt a Graecis hoc tractum esse vocabulum ἀπὸ τῆς ἀρετῆς, id est a virtute, quam scientiam vocaverunt.  Inter artem et disciplinam Plato et Aristoteles hanc differentiam esse voluerunt, dicentes artem esse in his quae se et aliter habere possunt; disciplina vero est, quae de his agit quae aliter evenire non possunt. Nam quando veris disputationibus aliquid disseritur, disciplina erit: quando aliquid verisimile atque opinabile tractatur, nomen artis habebit.