On the other hand, rational thought is not sufficient, and Van Helsing explicitly introduces this idea when first explaining to Dr. Seward the nature of Lucy's affliction. As he says, first you must forget the things that you believe you know. The men that give blood transfusions to Lucy do not do so as the result of careful cost-benefit analysis, and the dying smile of Quincey Morris is at least in part due to nonrational emotions, not just a careful calculus of one life for uncountable Un-Dead lives.
The link between rational thought and irrational thought is at the heart of economic analysis, with economists and psychologists probing what is possible for people to learn and decide. It is also at the heart of a Christian life. We must believe, but we are also charged to use all of our talents, including our rationality. The line between the two is everchanging. Given today's medical knowledge, I was shocked that the blood transfusions from 4 different men didn't kill Lucy -- surely at least one was not the same blood type. Actually, one wonders about the question of whether a vampire has to specialize in blood of a certain type, and whether how this would affect the population in a region. Presumably, if your local vampire is a type A, then the type B people have an advantage. Food for thought.