| 22322 | You can't define identity by same predicates, because two objects with same predicates is assertable [Wittgenstein] |
| Full Idea: Russell's definition of identity [x is y if any predicate of x is a predicate of y] won't do, because then one cannot say that two objects have all their properties in common | |
| From: Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921], 5.5302), quoted by Michael Potter - The Rise of Analytic Philosophy 1879-1930 53 'Ident' | |
| A reaction: [The Russell is in Principia] Good. Even if Leibniz is right that no two obejcts have identical properties, it is at least meaningful to consider the possibility. Russell makes it an impossibility, rather than a contingent fact. |
| 17594 | We can paraphrase 'x=y' as a sequence of the form 'if Fx then Fy' [Quine] |
| Full Idea: For general terms write 'if Fx then Fy' and vice versa, and 'if Fxz then Fyz'..... The conjunction of all these is coextensive with 'x=y' if any formula constructible from the vocabulary is; and we can adopt that conjunction as our version of identity. | |
| From: Willard Quine (Word and Object [1960], §47) | |
| A reaction: [first half compressed] The main rival views of equality are this and Wiggins (1980:199). Quine concedes that his account implies a modest version of the identity of indiscernibles. Wiggins says identity statements need a sortal. |
| 10797 | Substitutivity won't fix identity, because expressions may be substitutable, but not refer at all [Marcus (Barcan)] |
| Full Idea: Substitutivity 'salve veritate' cannot define identity since two expressions may be everywhere intersubstitutable and not refer at all. | |
| From: Ruth Barcan Marcus (Nominalism and Substitutional Quantifiers [1978], p.167) |
| 9848 | Content is replaceable if identical, so replaceability can't define identity [Dummett, by Dummett] |
| Full Idea: Husserl says the only ground for assuming the replaceability of one content by another is their identity; we are therefore not entitled to define their identity as consisting in their replaceability. | |
| From: report of Michael Dummett (Frege philosophy of mathematics [1991]) by Michael Dummett - Frege philosophy of mathematics Ch.12 | |
| A reaction: This is a direct challenge to Frege. Tricky to arbitrate, as it is an issue of conceptual priority. My intuition is with Husserl, but maybe the two are just benignly inerdefinable. |
| 9842 | Frege introduced criteria for identity, but thought defining identity was circular [Dummett] |
| Full Idea: In his middle period Frege rated identity indefinable, on the ground that every definition must take the form of an identity-statement. Frege introduced the notion of criterion of identity, which has been widely used by analytical philosophers. | |
| From: Michael Dummett (Frege philosophy of mathematics [1991], Ch.10) | |
| A reaction: The objection that attempts to define identity would be circular sounds quite plausible. It sounds right to seek a criterion for type-identity (in shared properties or predicates), but token-identity looks too fundamental to give clear criteria. |
| 11831 | The formal properties of identity are reflexivity and Leibniz's Law [Wiggins] |
| Full Idea: The formal properties of identity are the reflexivity of identity, and Leibniz's Law (if x is the same as y, then whatever is true of one is true of the other). | |
| From: David Wiggins (Sameness and Substance Renewed [2001], Pr.2) | |
| A reaction: Presumably transitivity will also apply, and, indeed, symmetry. He seems to mean something like the 'axiomatic formal properties'. |
| 16497 | Leibniz's Law (not transitivity, symmetry, reflexivity) marks what is peculiar to identity [Wiggins] |
| Full Idea: The principle of Leibniz's Law marks off what is peculiar to identity and differentiates it in a way in which transitivity, symmetry and reflexivity (all shared by 'exact similarity, 'equality in pay', etc.) do not. | |
| From: David Wiggins (Sameness and Substance [1980], 1.2) |
| 16498 | Identity cannot be defined, because definitions are identities [Wiggins] |
| Full Idea: Since any definition is an identity, identity itself cannot be defined. | |
| From: David Wiggins (Sameness and Substance [1980], 1.2 n7) | |
| A reaction: This sounds too good to be true! I can't think of an objection, so, okay, identity cannot possibly be defined. We can give synonyms for it, I suppose. [Wrong, says Rumfitt! Definitions can also be equivalences!] |
| 16502 | Identity is primitive [Wiggins] |
| Full Idea: Identity is a primitive notion. | |
| From: David Wiggins (Sameness and Substance [1980], 2.1) | |
| A reaction: To be a true primitive it would have to be univocal, but it seems to me that 'identity' comes in degrees. The primitive concept is the minimal end of the degrees, but there are also more substantial notions of identity. |
| 16015 | Problems about identity can't even be formulated without the concept of identity [Noonan] |
| Full Idea: If identity is problematic, it is difficult to see how the problem could be resolved, since it is difficult to see how a thinker could have the conceptual resources with which to explain the concept of identity whilst lacking that concept itself. | |
| From: Harold Noonan (Identity [2009], §1) | |
| A reaction: I don't think I accept this. We can comprehend the idea of a mind that didn't think in terms of identities (at least for objects). I suppose any relation of a mind to the world has to distinguish things in some way. Does the Parmenidean One have identity? |
| 16020 | Identity can only be characterised in a second-order language [Noonan] |
| Full Idea: There is no condition in a first-order language for a predicate to express identity, rather than indiscernibility within the resources of the language. Leibniz's Law is statable in a second-order language, so identity can be uniquely characterised. | |
| From: Harold Noonan (Identity [2009], §2) | |
| A reaction: The point is that first-order languages only refer to all objects, but you need to refer to all properties to include Leibniz's Law. Quine's 'Identity, Ostension and Hypostasis' is the source of this idea. |
| 16016 | Identity definitions (such as self-identity, or the smallest equivalence relation) are usually circular [Noonan] |
| Full Idea: Identity can be circularly defined, as 'the relation everything has to itself and to nothing else', …or as 'the smallest equivalence relation'. | |
| From: Harold Noonan (Identity [2009], §2) | |
| A reaction: The first one is circular because 'nothing else' implies identity. The second is circular because it has to quantify over all equivalence relations. (So says Noonan). |
| 16017 | Identity is usually defined as the equivalence relation satisfying Leibniz's Law [Noonan] |
| Full Idea: Numerical identity is usually defined as the equivalence relation (or: the reflexive relation) satisfying Leibniz's Law, the indiscernibility of identicals, where everything true of x is true of y. | |
| From: Harold Noonan (Identity [2009], §2) | |
| A reaction: Noonan says this must include 'is identical to x' among the truths, and so is circular |
| 6053 | Identity is as basic as any concept could ever be [McGinn] |
| Full Idea: Identity has a universality and basicness that is hard to overstate; concepts don't get more basic than this - or more indispensable. | |
| From: Colin McGinn (Logical Properties [2000], Ch.1) | |
| A reaction: I agree with this. It seems to me to follow that the natural numbers are just as basic, because they are entailed by the separateness of the identities of things. And the whole of mathematics is the science of the patterns within these numbers. |