Principle of individuation
thisness · haecceity
What makes two things that share all their properties two things rather than one. The candidates are matter, position, a primitive thisness, or nothing at all — and each answer commits you to a different account of what a particular is.
In practice
Any system that has to tell two identical records apart faces it and answers it with a surrogate key, which is the admission that nothing in the data itself does the job.
The common mistake
Thinking a list of properties can settle it. Every property you add can in principle be shared, so a purely qualitative answer either fails or has to rule out duplicates by assumption, which is the thing to be shown.
Imagine a universe containing nothing but two perfect iron spheres, alike in size, composition, temperature and every other quality, a mile apart.
Max Black put this case in 1952 and the intuition it pumps is hard to shake: there are two of them. Yet there is no property one has that the other lacks. Whatever is true of the first is true of the second, with the sole exception of being that one.
So what makes them two?
Why this is not a trick question
It matters because the alternative to answering it is denying the setup.
Leibniz held the identity of indiscernibles: if two things share every property, they are one thing. On that principle Black's universe is impossible, and the impossibility is metaphysical rather than practical. Leibniz was untroubled by this, since on his view no two substances are ever perfectly alike anyway — every monad reflects the whole universe from its own position, so a difference of position is a difference in nature.
The obvious rescue — that the spheres differ in where they are — is weaker than it looks. In a universe containing only the spheres, position is a relation between them and nothing else, so each is "a mile from a sphere just like me". That description is again shared. Space has not individuated them; it has only restated the problem with more geometry.
The candidate answers
Matter. Aristotle's answer, and Aquinas's: two things of the same kind differ because they are this parcel of matter and that one. Form makes a thing the kind of thing it is, and matter makes it this one. The difficulty arrives for anything without matter — angels, on the scholastic problem, or minds — which is why Aquinas concluded that each angel must be its own species, there being nothing else available to tell two of them apart.
Haecceity. Duns Scotus's answer: a primitive non-qualitative principle, thisness, which is not a property in the ordinary sense and cannot be shared. It is precisely what the spheres have separately. The objection is that it is the question restated in Latin — we wanted to know what makes this one this one, and the answer is its this-one-ness.
Scotus's defense is that the demand for a further explanation is the mistake. Not everything can be explained by something else, and if individuality is fundamental then a primitive is the right form of answer rather than a failure to give one.
Nothing — individuals are basic. On this view the question is malformed. Particulars are not built out of anything, so there is no constituent that does the individuating, and the spheres are two because particulars are simply what there are. This is now a common position and it treats the whole dispute as a residue of thinking of things as bundles.
Bundles plus a location. If a thing just is its properties — bundle theory — then you need something to keep two identical bundles apart, and spacetime is the usual candidate. Black's case was aimed directly at this, which is why it is stated in an otherwise empty universe.
What it decides elsewhere
This is not an isolated puzzle. It is the joint several other questions turn on.
If a thing is nothing but a collection of properties, then personal identity has no deep answer — there is no underlying bearer for the question to be about, which is the conclusion Hume reached from the other direction. If instead a particular has a primitive thisness, then it can in principle persist through a complete change of properties, and the question of whether you are the person in your childhood photographs has a determinate answer even where nothing qualitative survives.
The same choice governs substance, where it is the dispute about whether there is a bearer underneath the qualities, and universals, where the question is what two things sharing a property actually share.
It is worth noticing that databases answer it by fiat. When two rows agree on every column and still need to be two, a surrogate key is added — an identifier carrying no information except that it differs. That is haecceity, implemented, and the reason it is necessary in practice is the reason the philosophical problem does not go away.
Concept web
Open the full webQuestions
What is the principle of individuation?
Whatever accounts for two things being two rather than one when they share their properties. The main candidates are matter, spatial position, a primitive thisness called haecceity, or the view that particulars are basic and need no such principle.
What is Max Black's two spheres argument?
A thought experiment in which a universe contains only two exactly similar iron spheres a mile apart. They appear to be two things with no differing property, which challenges Leibniz's principle that things sharing all properties are identical.
What is haecceity?
Duns Scotus's term for a primitive, non-qualitative principle of individuality — the thisness that makes a particular the one it is. Critics say it restates the question; defenders reply that individuality is fundamental and a primitive is the right kind of answer.