Taken from opening paragraph of Labyrinths.
Obviously this line is open to interpretation. One interpretation might be that he's denying excluded middle. But he would probably claim to be anti-logic (which of course to a logician is incoherent). At the least, the Greeks believed in the concept of truth, and (probably) that there is such a thing. You don't have to read much Borges to get the sense he doesn't believe in truth in this sense, that is, in bivalent truth.