Non-Necessary
Substance
A non-necessary substance is a substance that isn’t
a necessary substance. It’s a substance that doesn’t
exist no matter what. These are
either mental substances or non-mental.
It is fairly uncontroversial among philosophers that there are non-necessary
substances. But there are philosophers here and there who question this for
various reasons.
It’s been questioned on the grounds that for any
substance, S, if there is a world in which S does not exist, then the
proposition that S possibly exists would not be true in that world because that
proposition would not even exist there. It would not exist because S would not
exist, and no proposition can exist if the thing it’s about does not exist. But
given the modal axiom S5, the possibility of S’s existing
should itself be necessary—true in every world. Therefore, there is no world in
which S does not exist. The problem with this argument is the assumption that a
proposition about a thing cannot exist unless that thing exists, too. This
assumption comes out false on my theory of propositions.
I’ve also heard of philosophers thinking that every
substance is necessary on the grounds that every fact must have an explanation
that necessitates it. In reply, I do
not see why an explanation should have to necessitate its explanandum.
Some explanations may merely be statistical in nature, for example. Also, it’s
not perfectly clear to me that every fact whatsoever must have an explanation.
Perhaps there could be facts concerning the spontaneous activities or actions
of mental substances that are wholly unexplained (though perhaps those actions
are governed by certain fixed boundaries).