In the context of career choice, a person's replaceability is the degree to which their role impact would vary should that position be occupied by the next most likely candidate.

80,000 Hours illustrates this idea with an example:

Suppose you become a surgeon and perform 100 life saving operations. Naively it seems like your impact is to save 100 people’s lives. If you hadn’t taken the job, however, someone else likely would have taken it instead. So your true (counterfactual) impact is less than the good you do directly.

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