There is a difference in my opinion between a
Sceptic
and a
Pessimist.
A Sceptic believes things won't work out of experience of failure, he has tried or has known of it being tried before and all he has discovered is failure. Basically all he is doing is extrapolating on history, just as you do every time you go for a drive in your car you extrapolate from previous experience that when you turn your car key the engine will start (this is subjective cos some of us have more reliable cars than others, but hopefully you get my drift).
A Pessimist will not even try it, even in face of knowledge that it will work he will say it won't work this time.
Therefore
A sceptic is basically someone who has seen the facts and has come to the conclusion that it won't work. And so is a rational person.
A pessimist is someone who is operating on instinct, and so is not rational (ie does not base his judgement on facts).