Abstract

Absolute velocities in Newtonian mechanics are commonly regarded as unmeasurable. Roberts (Br J Philos Sci 59(2):143–168, 2008) provides a justification for this thesis which appeals to the observational indistinguishability of boost-related models of Newtonian mechanics. Middleton and Murgueitio Ramírez (Australas J Philos, 2020) criticise his argumentation by pointing out that his analysis of the notion of measurement is too restrictive, and that, under a weaker analysis (based on counterfactuals), absolute velocities are measurable. Jacobs (Australas J Philos, 2020) opposes their view, arguing that, on a properly formulated counterfactual-based account of measurement, absolute velocities are unmeasurable. However, in his argument, observational indistinguishability plays no role, even though, intuitively, it is very relevant for the issue of (un)measurability. This paper’s aim is to bring observational indistinguishability back to this discussion. I build upon the observation that there is an analogy between possible views on knowledge and justification on the one hand, and on measurement on the other. In particular, I explore the distinction between externalist and internalist approaches. Counterfactual-based accounts are all externalist, whereas the observational indistinguishability of boost-related models becomes relevant if we are interested in an internalist concept of measurement.