Sage Essays on Reward and Punishment

I remember reading in your book once that when the sages say, "Whoever does... gains... / After all, this is..." and the like - this is not an absolute rule, and the intention is that a certain action influences a certain result, and there can always be other factors that influence the other side, like a law of nature. (Yes, it wasn't as clear as the original column, but I hope it's enough to mention).
Can you direct me to the column in question?