1. On terminology: Perry ([2015]) calls `additive' what I call extensive. I use `additivity' as Michell ([1999], pp.53-54) does: to state how the degrees of quantitative attributes relate to each other (so not to speak of how objects instantiate attributes).Measurementtheorists have typically called `extensive' attributes that we can measure using concatenation operations.
2. Note that physicists are also forced to think of even length or mass as `theoretical concepts' (versus observational properties) as soon as they go beyond the measurement of middle-size objects in laboratory conditions (Carnap [1966], pp.102-104).
3. On this point Michell is surely correct. He has emphasized throughout his work ([1990], p.155; [1999], p. 207; [2012a], p. 265) the lack of specificity-of a quantitative character-of psychological theories, alerting that this lack of detail entails that we are not properly able to test quantitative claims about psychological measurands.