  Why think this a genuine puzzle? There seem to be plenty of everyday cases where knowing some thing in no way prevents us from sometimes mistaking that thing for something else. One example in the dialogue itself is at 191b (cp. 144c5). It is perfectly possible for someone who knows Socrates to see Theaetetus in the distance, and wrongly think that Theaetetus is Socrates. The First Puzzle does not even get off the ground, unless we can see why our knowledge of X and Y should guarantee us against mistakes about X and Y. Who is the puzzle of 188a–c supposed to be a puzzle for?