1 Varieties of possibility and necessity

Recall Descartes’ conceivability argument that he is not his body:

1.

Everything that I can conceive clearly, distinctly, and without contradiction, is possible.

2.

Thus, if I can conceive that something exists without something else, then it must be possible for the former to exist without the latter.

3.

I can conceive of myself existing without my body.

4.

I can conceive of my body existing as a non-thinking thing.

5.

So it must be possible for me and my body to exist without each other; therefore,

6.

I am really distinct from my body, and can exist without it.

Let’s focus on the first premise: everything I can conceive clearly, distinctly, and without contradiction, is possible.

In general, to say that something is necessary is to say that that thing must be the case. To say that something is possible is to say that it could be the case. Finally, to say that something is contingent is to say that it’s neither necessarily true nor necessarily false. Necessity and possibility are usually taken to be inter definable: to say that something is necessary is to say that it’s not possible for that thing not to obtain.

In English, there are different things we can be saying when we talk about possibility and necessity. We will describe three of these in what follows:

Metaphysical possibility/necessity:

When we say that something is metaphysically possible, we are talking about the ways in which things could be. For instance, I have black hair, but it is metaphysically possible that my hair was a different color: if things had been different, for instance, if my genes were slightly different, the color of my hair could have been different too. Again, we use possibility in the metaphysical sense when we want to describe ways things could be.

Epistemic possibility/necessity:

When we say that something is epistemically possible, we are talking about what could be the case for all we know, or for all we believe. For instance, for all I know, it’s possible that my friend’s turtle is alive and happy. Note that something can be metaphysically possible without it being epistemically possible: it is metaphysically possible that I was born in Mars, but I know that I was born in planet Earth, so it’s not epistemically possible that I was born in Mars.

Is there anything that is epistemically possible but not metaphysically possible? Yes: supposed that, unbeknownst to me, the person called ‘Sarah’ and the person called ‘Jen’ are one and the same person. Since I don’t know this, it seems it’s epistemically possible (for me) that Sarah is not Jen. But since Sarah and Jen are actually the same person, it’s not metaphysically possible that they are different people!

We should be careful here: it’s one thing for it to be possible that there are two different people, one called ‘Sarah’ and the other called ‘Jen’. It’s a completely different thing to say that Sarah could be someone other than herself. The first is possible; the latter is impossible.

Physical possibility/necessity:

We defined epistemic possibility and necessity by using the expression for all I know. We can define other kinds of possibility and necessity in similar ways. For instance, we can define physical possibility as that which is possible given the laws of physics. Thus, everything that is compatible with the laws of physics that actually hold is said to be physically possible. Some things are physically impossible but metaphysically possible. For instance, that the law of inertia fails.

With these distinctions in place, we should be in a better position to evaluate Descartes’ argument. First of all, we need to find out which kind of necessity he must be talking about in order for his argument to be valid. Can he be talking about epistemic or physical possibility? No: his first premise clearly doesn’t hold if we’re talking about physical possibility, and the step from 5 to 6 doesn’t hold if we’re talking about epistemic possibility.

When Descartes talks about possibility in his argument, he is talking about metaphysical possibility. Having made this clarification, does his argument hold? Some philosophers have denied that the first premise holds, even when we’re just talking about metaphysical possibility. This is illustrated by the example of Sarah and Jen above. However, other philosophers have replied to this attack by saying that the premise establishes a link between conceiving things in the right way and possibility. Question: what could be this right way of conceiving things, and how could we distinguish it from the wrong way of conceiving things?

For the time being, let’s examine the merits of Descartes’ own version of dualism.

2 Descartes’ dualism

Descartes’ view about the nature of the mind is usually called substance dualism. It is so called because it posits two kinds of substances: mental and material—or physical, as we would now call them.¹

Descartes’ dualism attempts to recover the common sense intuition that our minds interact with our bodies: our mind can control our body at will, and our body can influence our minds by producing in it pains, hunger, and other sensations including sense perception. For this reason, Descartes’ view is also a version of interactionist dualism. The main tenet of interactionist dualism is simply that, though the mind and the body are different kinds of substances, they can interact with each other. The following remarks by Descartes show his commitment to this view:

Descartes’ view was forcefully attacked since the very beginning, which is why we will consider some objections raised by Princess Elisabeth of Bohemia.

3 Elisabeth’s challenges

Recall Descartes’ working definition of a body:

Moreover, Descartes has argued that he (his soul, his mind) can exist without a body, and without having any extension. This is what puzzles Elisabeth:

Descartes has claimed that the mind and the body causally interact with each other, and Elisabeth is asking for an explanation of how the interaction is supposed to obtain. Elisabeth challenges is as follows: causing a material thing to move requires being extended, but Descartes claims that minds are not extended. So how can minds cause bodies to move?

In another letter, Elisabeth presses the challenge: