I've been reading A very short introduction to literary theory. Just before talking a bit about Rousseau, the author argues that "western philosophy traditionally distinguishes between appearance and reality". I google for that and found Bradley's book: Appearance and reality from 1893. As the author said "traditionally", I thought it was much older than that. Is this the origin of this idea or is it actually older than 1893?
The distinction between appearance and reality is extremely ancient, much before written history. Any myth, any form of religion and world-view, is inherently bound with a distinction between appearance and reality.
Philosophy itself is often said to have been born out of ancient myth. The distinction between appearance and reality has been a constitutive part of philosophy, and right from the start. The earliest known philosopher in the West, Thales of Miletus (c. 600 BC) is reported to have theorized that "Water constituted (ὑπεστήσατο, 'stood under') the principle of all things". By that he is believed to have meant, that although there appear to be many unrelated things, and many unrelated events, in reality there is one principle, one ground, one explanation behind it all, and that one principle is identical with what we think of as water.
An additional source from Plato (429?–347 B.C.E.), from Theaetetus, 157e, discussing Theaetetus' proposed definition (in 151e) that "knowledge is nothing else than perception [ἄλλο τί ἐστιν ἐπιστήμη ἢ αἴσθησις]" :
Socrates : Let us, then, not neglect a point in which it is defective. The defect is found in connection with dreams and diseases, including insanity, and everything else that is said to cause illusions of sight and hearing and the other senses. For of course you know that in all these the doctrine we were just presenting seems admittedly to be refuted, because in them we certainly have false perceptions, and it is by no means true that everything is to each man which appears [τὰ φαινόμενα] to him; on the contrary, nothing is which appears.
It is woth noting that the examples of "false perception" used regard disease, madness and dreams... a lot of centuries before Descartes.
Plato's Simile of the Cave (c. 380 BC) is the earliest I know of. The inhabitants of the cave think the shadows cast by the fire are the real phenomena, but it isn't until they leave the cave and see things in sunlight that they discover the truth.