Richard Swinburne on metaphysical necessity

www.youtube.com/watch?v=KhJ2XaGbYZ4, 15m

I've asked the question whether there is anything meaningful about the idea of 'metaphysical impossibility', and concluded that it can be reduced to logical impossibility. https://benstowell.blogspot.com/2024/10/physical-possibility-vs-metaphysical.html

Swinburne says before the 1970s, the only notion of necessity was logical necessity where denying the truth of something entails a contradiction. Then Kripke and Putnam presented the example of denying "Water is H2O." Water is necessarily H2O, but denying that water is H2O does not entail a contradiction. So here you have something that is necessary but not logically necessary, and here is where we need the notion of 'metaphysically necessary.' Facts of essences, or essential facts, are metaphysically necessary. We can think of it in terms of possible worlds. In all possible worlds, water is H2O. It's impossible for water to not be H2O. And yet, there is no contradiction in saying water is H3O or some other chemical composition.

Swinburne rejects this. If by 'water' we mean 'H20', then denying 'H2O is H2O' does entail a contradiction. It is logically necessary for everything to be identical to itself. If by water we mean 'blue stuff in our rivers and oceans,' then 'water is blue stuff in our rivers and oceans' is not necessary. There are possible worlds in which water is not (perceived to be) blue or not in our rivers or oceans.

So the confusion arises from equivocating on essential vs non-essential properties. If we define water in terms of essential properties, then denying that water is H2O is a logical contradiction. If we define water in terms of non-essential properties, then it's not necessary for water to be that. So we do not have an example here of something that's both necessary and yet not logically necessary.

This reminds me of a very similar confusion, if not the very same, that motivates the Water/H2O / Superman/Clark Kent response to the objection against identity theory that identifying thoughts with brain states immediately violates Leibniz's Law. Joshua Rasmussen explains things nicely here: https://www.youtube.com/watch?v=KhJ2XaGbYZ4

Swinburne reduces metaphysical necessity to logical necessity too. Neat!