Euclid is merely the first whose work has survived to the modern day. If tradition is to be believed, Thales and Pythagoras provided proofs of non-intuitive results from intuitive one. Furthermore, Hippocrates of Chios wrote a systematic treatment starting with axioms. All three predated Plato.
That's a good point about Hippocrates, I'd forgotten about him. Do you have a source handy on Thales and Pythagoras? I don't doubt it, it's just a gap I should fill. So far as I remember, a proof that the square root of two is irrational came out of the Pythagorean school, but that's all I can think of. I hadn't heard anything like that about Thales.
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