Imagine going to hear a sermon, says Pascal. You are a reasonable and realistic person. You have great respect for the truth. But the preacher steps into the pulpit and he has an odd face or isn't well shaven or he isn't very clean. Suddenly it doesn't matter what great truths he has to proclaim.
Reason is overthrown by trivial details.
Or consider the fact that we are rational and yet mortal creatures. We are fragile and easily destroyed. The smallest thing, the smallest drop of water, a microbe or a virus, can end our lives. And so we are a paradox. The great philosophers tell us that we are rational creatures, capable of amazing thoughts and deeds.