‘To suppose two things indiscernible is to suppose the same thing under two names’. If the two universes cannot be told apart, according to PII, they are one and the same. Introducing Absolute space makes them different, this violating the principle.
Clarke says that the PII does not hold, for his ship example shows that there can be motion that is unobserved, just as absolute motion could be. Just because it is not observed, it does not mean it is not happening.
Leibniz clarifies his point here. He says that for objects to be different, it is not necessary for a difference to be observed, but only that it must be possible for the difference to be observed, which, in the ship case, is possible.
In kinematic shift (changing from one direction to another) however, there is no reference frame from which a difference could be seen. Leibniz holds that in such cases, PII does apply, and thus there is no difference, for we would never know of such a change, so why posit it? It is just empty talk and imaginative conceptions.
Leibniz’s arguments only have force if we accept PII. But we do seem to have reason to accept PII. Example in book goes along the lines of someone positing ‘nelectricity’, a charge of which everything carries, but this has no effect on anything. It is unobservable. We would probably say there is no nelectricity at all. Different nelectric charges are indiscernible so are nothing at all. The difference between the world as it was and the world with the new discovery is undetectable, hence does not exist.
Such is the case of absolute space, which has no effect on the world. It is unobservable, just like nelectricity. PII entails that we say AS is not real, for it is unobservable thus Newton has failed to tell us what absolute space and velocity is, seems he has not told us how to distinguish between them. This is a very Machian, realist position.
This empiricist interpretation if the PII suggests that it is meaningless and useless to posit absolute space seems there is no way we could observe any difference from one absolute space than another, thus this seems like a strong objection to Newton and Clarke.
A problem here arises; inertial effects. In Newton’s bucket and globes example, Clarke suggests that though, strictly speaking, absolute space is unobservable, we can get around PII by saying that the effects of absolute space or absolute motion are observable. We need absolute space to explain these inertial effects. Leibniz says very little on this point unfortunately. He says there is a difference between ‘true’ and relative motions. He talks about the ‘cause f the change being within the body itself’, but this tells us little, if anything about absolute acceleration in relational terms Leibniz dies before he could provide a further reply, but it is interesting to consider what this reply might have been like.
In static shifts, PII does seem to hold, but in dynamic shifts, the presence of inertial effects seems to posit absolute space, even though no motion can be seen from any reference frame.
In conclusion, PII seems very damaging to absolute space, but only in consideration of static shifts. In considering dynamic shifts, we PII does not hold, but that is not to say there are other relationist ways around the problem of inertial effects.
