Identity of indiscernibles

The identity of indiscernibles is an ontological principle which states that two or more objects or entities are identical (are one and the same entity) if they have all their properties in common. That is, entities x and y are identical if any predicate possessed by x is also possessed by y and vice versa. A related principle is the indiscernibility of identicals, discussed below.

A form of the principle is attributed to the German philosopher Gottfried Wilhelm Leibniz. It is one of his two great metaphysical principles, the other being the principle of sufficient reason. Both are famously used in his arguments with Newton and Clarke in the Leibniz–Clarke correspondence. Because of its association with Leibniz, the principle is sometimes known as Leibniz's law. (However, the name 'Leibniz's Law' is also commonly used for the converse of the principle, the indiscernibility of identicals (described below), which is logically distinct and not to be confused with the identity of indiscernibles.)

Some philosophers have decided, however, that it is important to exclude certain predicates (or purported predicates) from the principle in order to avoid either triviality or contradiction. An example (detailed below) is the predicate which denotes whether an object is equal to x (often considered a valid predicate). As a consequence, there are a few different versions of the principle in the philosophical literature, of varying logical strength – and some of them are termed "the strong principle" or "the weak principle" by particular authors, in order to distinguish between them.

Willard Van Orman Quine thought that the failure of substitutivity in intensional contexts (e.g., "Sally believes that p," "It is necessarily the case that q") shows that modal logic is an impossible project. Saul Kripke holds that this failure may be the result of the use of the disquotational principle implicit in these proofs, and not a failure of substitutivity as such.

Associated with this principle is also the question as to whether it is a logical principle, or merely an empirical principle.

Identity and indiscernibility

There are two principles here that must be distinguished (equivalent versions of each are given in the language of the predicate calculus). Note that these are all second-order expressions. Neither of these principles can be expressed in first-order logic.

1. The indiscernibility of identicals
   • For any x and y, if x is identical to y, then x and y have all the same properties.

     $\forall x \forall y[x=y \rightarrow \forall P(Px \leftrightarrow Py)]$

2. The identity of indiscernibles
   • For any x and y, if x and y have all the same properties, then x is identical to y.

     $\forall x \forall y[\forall P(Px \leftrightarrow Py) \rightarrow x=y]$

Principle 1 is taken to be a logical truth and (for the most part) uncontroversial. Principle 2 is controversial. Max Black famously argued against 2. (see Critique, below).

The above formulations are not satisfactory, however: the second principle should be read as having an implicit side-condition excluding any predicates which are equivalent (in some sense) to any of the following:

1. "is identical to x"
2. "is identical to y"
3. "is not identical to x"
4. "is not identical to y"

If all such predicates are included, then the second principle as formulated above can be trivially and uncontroversially shown to be a logical tautology: if x is non-identical to y, then there will always be a putative "property" which distinguishes them, namely "being identical to x".

On the other hand, it is incorrect to exclude all predicates which are materially equivalent (i.e. contingently equivalent) to one or more of the four given above. If this is done, the principle says that in a universe consisting of two non-identical objects, because all distinguishing predicates are materially equivalent to at least one of the four given above (in fact, they are each materially equivalent to two of them), the two non-identical objects are identical – which is a contradiction.

Critique

Symmetric universe

Max Black has argued against the identity of indiscernibles by counterexample. Notice that to show that the identity of indiscernibles is false, it is sufficient that one provides a model in which there are two distinct (non-identical) things that have all the same properties. He claimed that in a symmetric universe wherein only two symmetrical spheres exist, the two spheres are two distinct objects, even though they have all the properties in common.

Indiscernibility of identicals

As stated above, the principle of indiscernibility of identicals – that if two objects are in fact one and the same, they have all the same properties – is mostly uncontroversial. However, one famous application of the indiscernibility of identicals was by René Descartes in his Meditations on First Philosophy. Descartes concluded that he could not doubt the existence of himself (the famous cogito argument), but that he could doubt the existence of his body.

This argument is criticized by some modern philosophers on the grounds that it allegedly derives a conclusion about what is true from a premise about what people know. What people know or believe about an entity, they argue, is not really a characteristic of that entity. Numerous counterexamples are given to debunk Descartes' reasoning via reductio ad absurdum, such as the following argument based on a secret identity:

1. Entities x and y are identical if and only if any predicate possessed by x is also possessed by y and vice versa.
2. Clark Kent is Superman's secret identity; that is, they're the same person (identical) but people don't know this fact.
3. Lois Lane thinks that Clark Kent cannot fly.
4. Lois Lane thinks that Superman can fly.
5. Therefore Superman has a property that Clark Kent does not have, namely that Lois Lane thinks that he can fly.
6. Therefore, Superman is not identical to Clark Kent.
7. Since in proposition 6 we come to a contradiction with proposition 2, we conclude that at least one of the premises is wrong. Either:
   • Leibniz's law is wrong; or
   • A person's knowledge about x is not a predicate of x; or
   • The application of Leibniz's law is erroneous; the law is only applicable in cases of monadic, not polyadic, properties; or
   • What people think about are not the actual objects themselves; or
   • A person is capable of holding conflicting beliefs.

Any of which will undermine Descartes' argument.

A response may be that the argument in the Meditations on First Philosophy isn't that Descartes cannot doubt the existence of his mind, but rather that it is beyond doubt, such that it is impossible for a being with understanding to even consider doubting it. This much stronger claim doesn't resort to relational properties, but rather presents monadic properties, as the foundation for the use of Leibniz's law. One could expound an infinite list of relational properties that may appear to undermine Leibniz's law(i.e. Lois Lane loves Clark Kent, but not Superman. etc.) but nonetheless any approach focused on monadic properties will always produce accurate results in support of Descartes' claim and provide proof for Leibniz's Law.

See also

• Disquotational principle
• Identical particles, a similar idea in quantum mechanics
• Intensional fallacy

References

• Lecture notes of Kevin Falvey / UCSB

External links

• http://www.consciousentities.com/tactics.htm
• Stanford Encyclopedia of Philosophy entry