This is a purely moral judgement - you can argue, in the most extremal case, that every aspect of a person's behavior is purely "circumstantial". After all, your personality and capabilities are entirely shaped by circumstances like genetics and life experience.

Of course, we understand this is a pretty useless way of looking at things.

From a consequentialist standpoint, the incentives work out the best when we use the heuristic of assigning responsibility for low labor value to the person performing the labor, so that's the approximation I choose to use. You could choose to think that e.g. being very stupid is no one's fault, so a very stupid person should get paid as much as a brilliant doctor, but the net social consequences of this are pretty well explored and pretty bad.