The origin of proofreading

Pythagoras is wrestling with his theorem of ultimate reality. A bright young assistant offers his services as “proofreader”.

...

Assistant: Can I proofread it for you, Master?

Pythagoras: Well, you could pick up the obvious stuff like spelling errors, grammar errors, leaps of logic, inconsistencies in style and presentation.

Assistant: I can do all that.

Pythagoras: But can you see inside my head? The things I really should add. Or, the things I don’t think I want left in the text?

Assistant: Both of us should work on it. You’ll miss what the reader will see and I’ll miss what you think the reader should see.

Pythagoras: OK. Bring me the red pens, the blue pens, the white correcting fluid, and a couple of rulers. Let the reading of the proof, the "perfect" number, 1 + 2 + 3 + 4 = 10, hereby begin.
...