The Ego Trick (baggini, Julian) / quote 144 of 254
Quantitative versus qualitative identity
For philosophers, ‘identity’ comes in two main forms: quantitative and qualitative. Two or more things are qualitatively identical when interchanging them would make no difference. For instance, if you have two qualitatively identical bowling balls, it makes no difference which one you pick up when you attempt a strike. Such qualitatively identical objects, however, are not quantitatively the same: they are not literally the same object. If you put a mark on one of the balls, the other is unaffected. If you smash one, you do not smash the other. If one is in one place, the other can’t be literally in the same place too. When we talk about the identity of objects over time, it is this quantitative identity that we are interested in. We want to know if the very same object has persisted over time, not merely whether two objects at different times are indistinguishable.