Why is Hesperus necessarily Phosphorus?

Question

"Hesperus (the evening star) is Phosphorus (the morning star)" is one of Kripke's examples of necessary aposteriori, statements that are true necessarily if true at all, even if their truth can only be established empirically. Since this is a path from is to ought there is a catch. According to Kripke (in Soames's phrasing), "being non-identical is a relation that holds essentially of any pair it relates. So, we know apriori that if any objects... stand in this relation, then they have, or stand in, them in any genuinely possible circumstance in which they exist". "Water is H20" necessarily for the same reason, although here identity is applied not to singular objects but to "natural kind" of objects.

I understand the reasoning, what I do not understand is what drives it. What makes some properties/relations essential and others not? what do we gain by attaching "essential" to them, and "necessarily" to statements? For example, is "inertial mass=gravitational mass" necessarily? Like Hesperus and Phosphorus they appear in two ostensibly different situations, when measuring inertia and attractive force respectively, however every measurement to date produced identical results. It is as empirically solid as the identity of Venus's manifestations, and a postulate of general relativity. But is it necessary? and what does that mean in practice? What about "green is extended" that Quine puzzled over, certainly every manifestation of green we met or imagined was extended in space?

Generally, when we discover some persistent empirical coincidence, how are we to decide if it is necessary or just true? I can think of two possible approaches, a testing principle or a guiding principle, perhaps their is a third or they can be mixed.

1) Testing: necessity is relative to a set of presuppositions to be tested. We heuristically designate some relations as essential, and see what distribution of necessities/contingents obtains, then we test if it works. Obviously we can not test it directly, since possible worlds are empirically inaccessible, but perhaps attaching modal logic to scientific theories may prove fruitful/unhelpful in some ways. This is not the impression I get from Kripke, he seems to envision some absolute intuitions about essential and necessary, but were there any concrete proposals for testing them, from others perhaps?

2) Guiding: there is some methodological principle or a speculative model about how the world works that motivate essentialness. Perhaps, not a cut and dry prescription but solid enough to specify broad classes of properties/relations as definitely essential, and others as definitely not. If so, what is this principle, and why is it plausible?

Answer 1

I guess I would go with at testing procedure -- consistency with a stated or implied theory.

Necessity is a modality -- the collection of 'must' statements, construed as rigidly as possible.

Instead of Kripke's semantics, consider an older (and better) way of looking at modals.

Modal assertions are incomplete statements that gain meaning only when an appropriate context is affixed. So necessity is dictated by what part of your reality is considered accidental, and what part is considered 'the context in which you are operating'.

A necessity statement gains meaning only when a global theory is attached. At that point the consequences of the theory are necessary, anything that does not violate it is possible, and everything else is just true or false.

This style of modal reasoning is most obvious in the modality of obligation. Every 'should' statement only has implications when an ethics is attached. By the ethics of Aristotle we should not kill our parents. By the ethics of Manson we probably should.

A similar relativism applies to 'essence'. Pick your paradigm, and its basic terms dictate what is essential. For biology, acidity is an accident; for basic chemistry it may be essential; for physics, it is emergent.

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To the motivating statement by Kripke, any modern mathematician must consider it childish.

There is no 'essentially essential', there is always an equivalence relation: a definition of 'essential' which eliminates all other details and establishes the context in which one is working. In a broader context, the things we consider different are really the same, in a narrower context, we can look inside our equivalence classes and study the structure of the isomorphisms hiding information from us.

Answer 2

Are you reading Naming and Necessity or the paper Identity and Necessity?

If you're just looking at N&N, then I encourage you to look at the paper, because Kripke there gives a pretty slick argument for the necessity of identity that is perfectly straightforward.

It starts from Leibniz's Law, which Kripke (plausibly) takes a kind of implicit definition of the very notion of identity:

(1) for all x, y (if x = y, then (if Fx, then Fy)) [premise]

The only other premise we need is the perfectly obvious truth that:

(2) for all x, it is necessarily the case that x = x. [premise]

The clever trick is that we instantiate (1) with the property of being necessarily identical to x for the dummy predicate F to obtain:

(3) for all x, y (if x = y, then (if necessarily x = x, then necessarily x = y)). [from 1, by universal instantiation.]

And now, we universal instantiate (3) to get:

(4) if x = y, then necessarily x = y [universal instantiation (3)]

and then:

(5) necessarily x = y [from 2, 4, modus ponens].

that's the argument that identity is necessary. Now the other philosophical question is what does the necessity of identity show us? That question is much more open ended and difficult to answer. Certainly the immediate consequence of Kripke's argument was to pose some difficulties for physicalist philosophers of mind in the 60s (who thought that the mind = the brain's activity, but that this was merely contingent. NOTE this isn't an argument against physicalism, just one particular flavor of it popular in the 50s-60s.)

The issue about "essentialism" is somewhat different. Kripke also believes that the names of natural kind terms are "rigid designators", which refer to the same kind necessarily. Whether Kripke is right that natural kinds terms are rigid designators is a separate issue from the question whether identity is necessary or contingent.