Right, and that is just what natural-science does.
It begins with the base-assumption that science will only deal with natural explanations for natural processes.
Science leaves supernatural phenomenon & explanations to other categories of philosophy, i.e., theology.
But I must also mention, this was originally intended to be a methodological, not ontological or metaphysical assumption.
Our forefathers believed that scientists would take off their methodological smocks when they leave the lab for the night and then go home to their families where they'd thank God for dinner:
It's much more than unreasonable, it's strictly verboten -- by definition, when you admit supernatural processes or explanations, then you are no longer working in science, but in some other field, such as theology, miracles or, dare I say, magic.
If we circumscribe the object of your study in such a way that prohibits a complete explanation then science doesn't give the whole picture. It just gives it's natural aspects.
And who is to say? If you get to choose what science is, you're not exactly beginning with a blank slate. That choice functions as a presupposition, and you've begun as a philosopher or theologian.
Well, there's the crux.
Obviously there are definitions of what constitutes a good chance that has nothing to do with the probability of it actually happening.