On the distinction between SI base units and SI derived units

This letter discusses the criteria that should be considered when deciding whether or not the distinction between base units and derived units in the International System of Units (SI) is useful, a debate that has been reignited following the 2019 revision of the SI. It is proposed that the correct question to ask is whether the concept of independent quantities, and the dimensional analysis it allows, remains useful—since that is the ultimate origin of the distinction between SI base units and SI derived units. Furthermore, the letter explains that this distinction is not related to whether the defining constants used in the revised SI after 2019 are any more fundamental than they were previously, nor whether the links between defining constants and SI base units are any less direct than they were prior to 2019. Finally, an opinion is offered that the concept of quantity dimensions and dimensional analysis remains useful, and certainly does no harm.