The Ego Trick (baggini, Julian) / quote 145 of 254
Leibniz's law
Quantitative identity is governed by a strict logic, summed up in what has become known as Leibniz’s law. In essence this says that if x and y are the same object, then what is true of x at any given time is also true of y at that same time. This is why two merely qualitatively identical bowling balls are not the same ball. If it is true of the first that it is on top of the second, then it cannot also be true of the second that it is on top of the first. If, however, I am told that ball x has the same size, weight, colour, composition and location as ball y, at the same time, then I know ball x is ball y. It’s the same logic that enables you to know that if Colonel Mustard was in the same room as the murderer, and that the only people in the room were the murderer and the victim, and that Colonel Mustard is not the victim, then Colonel Mustard is the murderer. The logic is unavoidable.