Which number should replace the question mark?

One day the great philosopher came upon an acquaintance who ran up to him excitedly and said, "Socrates, do you know what I just heard aboutone of your students?"
"Wait a moment," Socrates replied. "Before you tell me I'd like you topass a little test. It's called the Test of Three."
"Three?"
"That's right," Socrates continued. "Before you talk to me about my student let's take a moment to test what you're going to say. The first test is Truth. Have you made absolutely sure that what you are about to tell me is true?"
"Oh no," the man said, "actually I just heard about it."
"All right," said Socrates. "So you don't really know if it's true or not. Now let's try the second test, the test of Goodness. Is what youare about to tell me about my student something good?"
"No, on the contrary..."
"So," Socrates interrupted, "you want to tell me something bad about him even though you're not certain it's true?"
The man shrugged, a little embarrassed.
Socrates continued. "You may still pass though, because there is a third test - the filter of Usefulness. Is what you want to tell me about my student going to be useful to me?"
"Well it....no, not really..."
"Well," concluded Socrates, "if what you want to tell me is neither True nor Good nor even Useful, why tell it to me at all?"
The man was defeated and ashamed. This is the reason Socrates was a great philosopher and held in such high esteem.
It also explains why he never found out that Plato was having an affairwith his wife.