My best literal-ish interpretation "In the fields of observation (testing?) chance favors only the prepared spirits." It's interesting to me the explicit "only" construction, as opposed to the implied version in the typical English versions.
I'm not a native French speaker. It's quite possible that minds would be better here. I don't really know; I think the distinction can frequently be subtle, at least in my conception of the two. Depending on the situation, I might consider either to be a strict subset of the other, which means I'm sometimes equivocating.
