 The texts are sufficiently ambiguous to permit dissent, but arguably Leibniz intends that one manifestation of derivative active force is what he calls vis viva – ‘living force’. Leibniz had a famous argument with the Cartesians over the correct definition of this quantity. Descartes defined it as size times speed – effectively as the magnitude of the momentum of a body. Leibniz gave a brilliant argument (repeated in a number of places, for instance Section 17 of the Discourse on Metaphysics) that it was size times speed2 – so (proportional to) kinetic energy. If the proposed identification is correct then kinetic energy quantifies derivative active force according to Leibniz; or looked at the other way, the quantity of virtus (another term used by Leibniz for active force) associated with a body determines its kinetic energy and hence its speed. As far as the authors know, Leibniz never explicitly says anything conclusive about the relativity of virtus, but it is certainly consistent to read him (as Roberts 2003 does) to claim that there is a unique quantity of virtus and hence ‘true’ (as we have been using the term) speed associated with each body. At the very least, Leibniz does say that there is a real difference between possession and non-possession of vis viva (e.g., in Section 18 of the Discourse) and it is a small step from there to true, privileged speed. Indeed, for Leibniz, mere change of relative position is not ‘entirely real’ (as we saw for instance in the Correspondence) and only when it has vis viva as its immediate cause is there some reality to  it.[12]  An alternative interpretation to the one suggested here might say that Leibniz intends that while there is a difference between motion/virtus and no motion/virtus, there is somehow no difference between any strictly positive values of those quantities.