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In some instances, yes, it could be considered a paradox. But in general, I would say it is not. My reasoning behind this is that hate is a feeling, you can feel something but be unsure as to why. So, essentially, he can indeed hate something he is not aware of, simply for the fact the details had slipped his mind, but the feeling happens to linger. In which case, he knows something was there and so he would be frustrated by the fact he cannot remember what it is.