Your premise is incorrect, there are at least two other soruces<br />
Plato is only the most famous
Someone had a trial after all...<br />
The question is how much what Plato wrote were really Socrates' words
... then change his name to Jesus Christ and be done with it!<br />
There's no proof of his existance other than the Bible.
Just ask Bill & Ted..they`ll confirm it
And the "proof" that Plato lived is what? HIS writing? Maybe someone made up PLATO to avoid persecution.<br />
Are YOU real or is someone making YOU up?
The thought here is that a real philosopher did exist especially concerning his earlier dialogues, as opposed to his later dialogues being influenced by Plato's own views, which strayed from the original philosophical views, whose name Plato could've likely used as his won pseudonym to cover up his identity due to fear of stirring the wrong feathers
a lot of people think that there was a real socrates, but that later writers also used him as a "character" or exposition device. there was not the info-checking technology at that time ;)
There are other sources of his existence. As well as his trial in other wrtings of the times. As well as the writings of his students.