Led by Leibniz
About this course
The fifth module of the Metaphysics paper takes up identity — what it is for something to be one and the same thing, both at a moment and over time. It is led by Leibniz, who gave the subject its two governing principles and who argued, against Locke, about what makes a person the same person across a life.
Leibniz's instinct is that a tangle in reasoning is a sign of an unclean concept: separate the ideas properly and the dispute can almost be calculated. So the module begins by prising apart what "same" runs together — being one thing versus being exactly alike — and his two principles: that identical things share all their properties (near-undeniable), and the far bolder converse that things sharing all their properties are identical (there are, he insists, no two leaves alike). From there it builds to the hard cases: the ship whose every plank is replaced and then rebuilt from the originals; the poker that is straight at noon and bent at midnight, which forces a choice between a thing's enduring whole and its having temporal parts; and finally personal identity — body, soul, or memory? — where Leibniz takes on Locke directly.
The student leaves able to wield numerical identity and Leibniz's Law, to argue the Ship of Theseus and the endurance/perdurance dispute, and to weigh the criteria of personal identity with their classic objections — carrying the open questions into Hume's attack on the self and on necessity. They read Leibniz, Locke's chapter on identity, and the New Essays directly.
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Led by Leibniz
The question
What makes something one and the same thing — and what makes it the same thing *still*, after it has changed? Leibniz separates being numerically one from being merely alike, and offers two principles: that identical things must share every property, and, more boldly, that things sharing every property must be identical — so that the world never exactly repeats itself. The module then turns to persistence: how the ship survives the replacement of all its planks (and what happens when the old planks are rebuilt into a second ship), whether a thing lasts by being wholly present at each moment or by having parts spread through time, and — the case that matters most — what makes you the same person as the child you were and the person you will be tomorrow.