Locke on identity Notes for November 9

Main points

This is our first day on metaphysics. Our first metaphysical question is: what are the necessary and sufficient conditions of identity? Specifically, we’re interested in the identity of persons

We discussed the first half of Locke’s chapter on “Identity and Diversity.” This covered Locke’s views about identity in general, the identity of masses of matter, the identity of living things like plants and animals, and his initial statement of his view about the identity of persons.

The position Locke is driving towards is that the conditions for the identity of a person are different from the conditions for the identity of masses of matters, animal lives, or immaterial souls. I identified this in two ways: by contrasting Locke’s view with the immaterialist and materialist alternatives and by looking very closely at sections 9 and 10.


It’s no easy task to formulate questions about identity properly. Suppose you ask “What is involved in one’s thing being identical with another?” The appropriate answer is: nothing. One thing can’t be identical with something else. Or suppose you ask “what is involved in one thing’s being identical with itself?” Here, it’s hard to understand why there is a question. If I knew what the thing we were talking about was, I would pretty much have my answer. And if I didn’t, there would be little prospect of having an answer.

We usually ask questions about identity when some sort of change is involved. Would item I remain the same if it changed in way C or would it be different? Since change takes place over time, we can usually point, in a metaphorical way, to items at different points in time. So we can ask whether an item that exists at one time is identical with one at a different time. That way of putting the question doesn’t already provide an answer, unlike the ones I canvassed in the previous paragraph.

According to Locke, what is involved in one mass of matter remaining the same over time is that none of the bits of matter that make up the mass change. We have the same mass of matter now as we had last week only if the masses have exactly the same atoms. That is a necessary condition of their being the same mass of matter. A sufficient condition probably requires that none of the bits of matter have left the mass in the meantime.

A mass of matter can remain the same if it undergoes some changes. Say we have a mass of matter composed of white rocks. You can shine a red light on it, such that it will look red, and it will be the same mass of matter as it was when it looked white. It will be the same mass of matter if we put it in a truck and move it from California to Oregon, provided we are very careful and don’t shake any molecules off or allow any to join in.

Qualitative vs. numerical identity

The distinction between qualitative and numerical identity is extremely important.

Two things are qualitatively identical if all their qualities are the same. Strictly speaking, we should say something like “all their intrinsic qualities are the same.” Why the qualification? Well, you could have two balls sitting side by side that would have exactly the same qualities except for one: their spatial position. The point is that the two balls themselves are exactly alike: if you turned around and I switched them, you wouldn’t be able to tell them apart. That’s what I mean in calling them qualitatively identical.

The example also illustrates numerical identity. There are two balls. Even though they are qualitatively identical, they are numerically distinct. You could destroy one and the other would remain intact.

This is going to be important when we talk about people down the line. Would someone who has all of your thoughts and memories be numerically identical with you or merely qualitatively identical?

This page was written by Michael Green for Problems of Philosophy, Philosophy 1, Fall 2009. It was posted November 10, 2009.
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