[189c]
when a person makes an exchange in his mind and says that one thing which exists is another thing which exists. For in this way he always holds an opinion of what exists, but of one thing instead of another; so he misses the object he was aiming at in his thought and might fairly be said to hold a false opinion.
Theaetetus
Now you seem to me to have said what is perfectly right. For when a man, in forming an opinion, puts ugly instead of beautiful, or beautiful instead of ugly, he does truly hold a false opinion.
Socrates
Evidently, Theaetetus, you feel contempt of me, and not fear.
Theaetetus
Why in the world do you say that?
Socrates
You think, I fancy, that I would not attack your “truly false”
