Kant's Categorical Imperative states: Act as if the maxim of your action were to become through your will a be general natural law. This according to Kant, is a rational ethical truth.
If you were arguing with a thorougly selfish person, could you show him or her that it was somehow irrational to cheat on other people while hoping that others will not cheat on you.
Q: What if everybody did?
A: I'm not everybody! I'm me, and I'm looking out for #1.
You might call such a person a jerk, but could you call him or her irrational?
I did something on this a few months back, to which I link.