Answer

I use these terms in somewhat different senses. Strictly speaking, talking about probability in the mathematical sense requires defining a sample space and well-defined events within it. But you can talk about reasonableness even without that. In that sense, an event with a high probability is reasonable, but not everything that is reasonable has a probability that can be calculated. For example, the principle of causality seems reasonable to me, but I do not know how to calculate the probability that it is true.