cartesian doubt - is the doubt that anything exists.
Descartes solves this through the 'cogito' ie 'i think therefore i am'
he then goes on to 'prove' that not only the 'I' exists, but certain 'clear and distinct ideas' (ie God) and he then posits that probably od wouldn't want us all to be deceived by demons, so relity, rpobably, is real..
there are many problems with the cogito, such as that pointted out that 'think therefore I am' does not follow'i think..therefore there is thought' is all that is actually proved.
in answer to he OP - the art of being a great Philosopher is essentially to stand up tall, an write the most coherently, boldly wrong piece of thought you can.
recent philsophers have been less willing to be wrong, which is why none of them qualify as greats.
In plato's specific case, the mouthpiece of his books, Socrates, is a really great thinker 'wise is he who knows he knows not' etc... who knows whether Platowas much cop by himself? One suspects so, and that he uses his 'Socrates' to put forth ideas that are really his.
oh yes and Artistotle was indeed a bugger for the bottle.