He has a clear head and can tell right from wrong.
Tips
history
Rooted in 《礼记·中庸》's canonical five-step learning: 博学之、审问之、慎思之、明辨之、笃行之 - study broadly, question carefully, think deeply, discriminate clearly, act firmly. 明辨之 is the ancestor of this idiom.
memory
是 = right/correct, 非 = wrong. 明辨是非 = 'clearly distinguish the 是 from the 非'. A four-character moral compass.
