The sentence is trying to explain that an irrational fear is not something that can be addressed through logic or rational thought. It could be anything, but I will use a fear of clowns or of spiders as an example. Probably, you have never been hurt by one, but their appearances, what they do, etc., creates a fear within your mind that you cannot control. So, there is no real reason for you to be afraid, to have fear, but you do, so that "fear does not make sense to begin with" right? My co-worker, who is six feet tall and weighs approx. 200 lbs, is afraid of mice! Can a mouse hurt him? No, but he is afraid nevertheless. Understand? I hope this helps you. J.