174 ARGUMENTS: MONOTHEISTIC CONCEPTIONSde re there corresponds a necessity de dicto expressible in a corresponding
conditional statement. If things have essences, then insofar as our
concepts of things are accurate regarding their essences, those concepts
will enable us to see de dicto as well as de re necessities.
This account of necessity is controversial and a defense of it would be
lengthy and complex,^6 and though I think it is also successful the purpose
of presenting it is simply to explain what is meant here by logical
necessity. It is an appropriate meaning for use in discussing the
ontological argument.
Purely conceptual proofs and the Ontological
Argument
A purely conceptual proof of God’s existence is an argument that is valid, has
only necessary truths as premises, and has God exists as its conclusion. Such
a proof that extends our knowledge – which of course is what is sought – will
satisfy (at least something like) the other conditions noted above.
The most famous attempts to provide such a proof constitute various
varieties of the Ontological Argument – an argument offered (among many
others) by St Anselm in medieval times, Descartes in the modern period, and
Alvin Plantinga in contemporary philosophy. In coming to understand this
sort of argument, we begin with two definitions:
Definition 1: X is a logically necessary being = X exists is necessarily
true (X does not exist is self-contradictory).
Definition 2: X is a causally necessary being = X exists is true and
logically contingent, and X is caused to exist is self-
contradictory.
A logically necessary being has not causally but logically necessary existence
- it exists and it is not possible that it not exist.
Four objections to the notion of logically necessary existence
There are various objections to the very idea of logically necessary existence.
Here are four of the most common:
1 All necessary propositions are conditional – they have a structure
properly expressed in an If A then B form.