In this paper I take a close look at the SI base quantity “amount of substance”, and its unit, the mole. The mole was introduced as a base unit in the SI in 1971, and there is currently a proposal to change its definition. I argue, first, that the current definition of the mole creates a certain ambiguity regarding the nature of the quantity “amount of substance”. I then evaluate some of the criticisms this ambiguity has prompted. Finally I look at how the new proposal affects the ontological status of “amount of substance”.

This paper is under review. Please do not cite without permission. (pdf)