Philosophy

5

LECTURE: The conceivability argument

One of the main arguments for the kind of dualism associated with Descartes is known as the “conceivability argument” (or sometimes the “modal argument”). Descartes gives an early version of the argument in his Sixth Meditation, and I discuss a contemporary version in chapter 2 of my book Philosophy of Mind. Contemporary versions are similar in spirit to Descartes’ version, though they often differ in the details. One of the key influences on recent thinking about the argument is the work of the contemporary philosopher Saul Kripke, especially some of his general ideas in metaphysics and philosophy of language. So, before looking at the conceivability argument itself, let’s say a little about Kripke’s ideas.

Better call Saul. The man himself:

Possible worlds

The ideas in question might at first glance not seem relevant to the mind-body problem, but we’ll see the relevance before long. Start with the notion of a possible world. A “possible world,” in the sense in which that expression is used in contemporary philosophy, is a complete way that things might have been.

What does that mean, exactly? Well, consider some examples. In the actual world, I teach PHIL 8 this semester. But things could have turned out differently. For example, I could have decided instead to teach only classes in Critical Thinking. Or I could have decided to take a sabbatical this year rather than teach at all. That’s not what actually happened, but it could have happened had things gone differently. There’s nothing about those scenarios that makes them intrinsically impossible – the way that a round square, for example, is intrinsically impossible, since it is self-contradictory. So, you might say that there is a “possible world” where I didn’t teach PHIL 8 this semester, even though it’s not the actual world.

There are all kinds of other possible worlds in this sense, including ones that are different from the actual world in far more radical ways – possible worlds where World War II never happened, possible worlds where the laws of physics were different, possible worlds where life existed on every planet, and so on. These are ways things could have been, even though they are not actually that way.

Now, some suggest that we can understand the alethic modalities in terms of possible worlds. That’s a fancy label for the different ways that a statement can be either true or false. It can be a necessary statement, or a possible one, or a contingent one, or an impossible one.

A necessary statement, on this analysis, is one that is true in every possible world. It is one such that, no matter how different things could have been in other ways, the statement would still have been true. Truths of arithmetic are stock examples. “1 + 1 = 2,” “2 + 2 = 4,” and the like are true of necessity, or in every possible world.

A possible statement, on this analysis, is one that is true in at least one possible world. For example, “World War II never happened” is a possible statement in this sense. It is false in the actual world, but there are other possible worlds where it is true.

A contingent statement, on this analysis, is one that is true in some possible worlds and false in others. For example, “Feser teaches PHIL 8 during the fall 2020 semester” is true in the actual world, but false in possible worlds where I decided to teach something else. (How does this differ from a possible statement? Well, some statements are both possible and necessary. For example, 1 + 1 = 2 is a possible statement – it is true in at least one possible world – but also necessary. But it is not contingent, precisely because it is necessary. So, though every contingent statement is possible, not every possible statement is contingent.)

Routes to various possible worlds

An impossible statement, on this analysis, is one that is false in every possible world. For example, “2 + 2 = 5” or “Round squares exist” would be impossible in this sense.

Rigid designators

Now, Kripke applies these ideas to the analysis of what he calls a rigid designator. A “designator” is a term that refers to something. For example, “Feser” refers to or designates me; “Los Angeles” refers to or designates a certain city: “the front door” refers to or designates a certain physical object; and so on. A “rigid designator,” Kripke says, is an expression that refers to the same thing in every possible world in which the thing in question exists at all. Examples would be proper names (like “Edward Feser”) and natural kind terms, i.e. terms that refer to naturally occurring kinds or classes of things (such as “water,” “gold,” “lead,” etc.).

OK, so what does all of that mean? Start with a specific example, like the term “water.” The way such a term enters the language, in Kripke’s view, is as follows. “Water” starts out as a kind of bare label for a certain kind of stuff. We might imagine, for example, that when the word came into use, what happened is that people said “Let’s call this stuff ‘water’” (as they pointed to a river or to rain, say). No doubt this is not exactly how it actually happened in the case of “water,” but let’s pretend it did just to convey Kripke’s basic idea. The word enters the language like a kind of verbal “sticker” that we slap on to something.

Now, when later language users use the term, what they mean by “water” is, essentially, “that particular stuff we have always been calling ‘water’ going back to the particular time when the word was first applied.” Later use of the term involves an implicit reference back to this original “naming” event.

Now that’s true, in Kripke’s view, even when we start talking about other possible worlds. For example, suppose we entertain the idea of a possible world where cars run on water instead of gasoline. What we’re really entertaining, Kripke argues, is a possible world where the stuff that in the actual world is called “water” is what cars run on. In that sense, “water” is a rigid designator. Whatever possible world we might be describing when we use the term “water,” it refers rigidly, to the very same stuff which in the actual world has been called “water” going back to the time the word was introduced. That’s just the way that natural kind terms and proper names work, in his view.

Not every term is a rigid designator. An expression like “the guy who teaches PHIL 8” refers to me in the actual world, but to someone else in another possible world where some other professor teaches the class. But natural kind terms and proper names are rigid designators.

Identity statements

OK, we’re almost to the point of seeing what all of this has to do with mind and body. But first, one more background idea. Consider identity statements, which are statements having the form “A = B” – for example, “water = H2O,” “the guy typing this = the guy who teaches PHIL 8,” and so on. Specifically, consider identity statements in which there is a rigid designator on each side of the = sign (which is true of the first of those two examples, but not the second).

An identity statement

Kripke argues for the following principle:

Kripke’s principle of identity: identity statements involving rigid designators are, if true at all, necessary truths, i.e. true in every possible world

The basic idea is this. If A refers to the same thing in every possible world, and B refers to the same thing in every possible world, then if A = B in the actual world, A = B in every possible world. For example, if “water” refers to the same thing in every possible world, and “H2O” (which is also a rigid designator) refers to the same thing in every possible world, then if water and H2O are the same thing in the actual world, then they must be the same thing in every possible world.

A statement like “water = H2O,” then, is either necessarily true – true in every possible world – or it isn’t true at all. That’s just how such statements work, given the way rigid designators work.

Now, this gives us a way to determine the truth or falsity of an identity statement containing rigid designators. Ask yourself: Could there, at least in theory, be a possible world where A exists but B does not – where A ≠ B? If so, then it must be false that A = B. For if it were true that A = B, there should be no possible world, not even one, where A exists and B does not.

The conceivability argument

Now, that brings us at last to the “conceivability argument” for dualism, which these days is sometimes formulated in a way that makes use of Kripke’s ideas. (Though Descartes himself did not appeal to Kripke, since he lived centuries before Kripke was born!) It is also sometimes called the “modal argument” for dualism because it appeals to what is possible, necessary, etc. where mind and body are concerned (recall the phrase “alethic modalities”).

Consider an identity statement like “mind = body” or “mind = brain.” All of the terms in these statements (“mind,” body,” “brain”) are natural kind terms. So, if statements like this are true at all, they would have to be necessary truths, true in every possible world. There should be no possible situation in which the mind exists but the body does not, or in which the mind exists but the brain does not. That follows from Kripke’s principle of identity.

Jars full of minds? Or just brains?

https://www.youtube.com/watch?v=K0lwfYXvnHE

Alvin Plantinga on the modal argument for dualism:

https://www.youtube.com/watch?v=WOTn_wRwDE0

Carneades.org on Kripke and rigid designators:

https://www.youtube.com/watch?v=h0GNeCRdx9s