What is rational is subject to a bit more controversy than how truth is understood. Accordingly I am just going to gloss over some ideas that I tend to believe about what it means to be rational.
First I think I would distinguish the irrationality of people who are insane. It seems to me that this sort of irrationality is beyond the person’s control. We are trying to be rational. To the extent we are trying to become rational we to some extent believe we have some control over this. Indeed I would say that and important part of being rational recognizing what is beyond our control and acting accordingly.
I also tend to think of being rational in terms of game theory. That is make rational or irrational choices. This I think can extend to what beliefs we hold – to the extent we can choose them.
So just a quick outline of what it means to be rational would be:
1) Not be illogical
2) Not be overly concerned with things beyond your control
3) Make choices based on analysis of the risks and benefits of your alternatives
4) Rational people tend not to go on emotion.
Some people will notice that I do not include Clifford’s claim “it is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence.” Frankly this claim has always seemed self-defeating and therefore in violation of the first principle I outlined – don’t be illogical.
That is, I do not have sufficient evidence to believe “it is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence” and so believing that principle would violate it.
This is just an off the cuff outline. Perhaps we could add some other concepts to this outline. For example I think rational people consider evidence for and against their position. But I think to the extent there is a difference between being “rational” and being “reasonable” I would say that belongs more along the lines of being reasonable, but think it could fit here as well. And the terms reasonable and being rational are very close anyway.