172 ARGUMENTS: MONOTHEISTIC CONCEPTIONSdepends on our language or our conventions. Such truths are true under all
conditions, and hence true in all possible worlds and across all possible
cultures.
The prime example of a necessary truth in the history of philosophy is
the principle of non-contradiction, which, as Aristotle said, is a principle
both of thought and of things. It can be expressed as Necessarily, no
proposition is both true and false or as Necessarily, no thing can have
logically incompatible properties. These ways of putting the principle are
mutually entailing. If something could have incompatible properties, then
propositions could be both true and false. If propositions could be both true
and false, then the proposition that a thing had incompatible properties
could be true and so a thing could have incompatible properties.
Many contemporary philosophers, using the language of some medieval
philosophers, distinguish between necessity de dicto and necessity de re –
necessity in speech and necessity in things. Suppose that Tony is a local
barber and consider two sentences about him:
A Necessarily, Tony is a person.
B Tony is necessarily a person.
These sentences express different propositions. What A says is this:
A1 Tony is a person, and it is logically impossible that Tony not be a
person.
This entails:
A2 Necessarily, Tony exists.
Of course A2 is false; however much he and his skills might be missed,
Tony does not enjoy logically necessary existence. There are possible
conditions in which, or possible worlds in which, Tony would not exist. No
doubt for a very long time our own world was Tonyless.
What B says is:
B1 Tony is a person, and anything that is a person is a person so long as it
exists at all; being a person entails being a person necessarily because
being a person (unlike being a Democrat or having false teeth) is an
essential property of anything that has it.
To embrace the idea that there are necessities de re one must think that there
are essential properties or essences. B1, and hence B, is true if this idea is true;
if there are essential properties or essences, being a person is among them.