Substance Dualism (Part One): The Basic Argument

Substance dualism is an enduring theory within the philosophy of mind. According to substance dualists the mind is ontologically distinct from the body.

With the rise of neuroscience in the 19th and 20th centuries, the theory appeared to become less tenable: the evidence seemed to point, overwhelmingly, toward a deep connection between mind and brain. But despite the continued presence of this evidence, a satisfactory theory of mind-brain relations remains elusive. As a result, my current sense is that substance dualism may be due to undergo something of a recrudescence. Thus, those of us with an interest in the philosophy of mind would be well-advised to understand the arguments that can be mustered in its support.

Fortunately, William Jaworski, through the medium of his recent book Philosophy of Mind: a Comprehensive Introduction, is on hand to assist. I’ve mentioned and recommended this book before. In this post, I will run through Jaworski’s presentation of the basic argument in favour of substance dualism. In future posts, I’ll consider the various criticisms and responses to this argument.



1. The Basic Argument for Substance Dualism
As is the case with many of the most intractable philosophical arguments, the argument for substance dualism can be stated with admirable brevity. [Note: In the argument “we” just means “persons or minds”]:

• (1) If we can exist without bodies, then we cannot be bodies.
• (2) We can exist without bodies.
• (3) Therefore, we cannot be bodies.

As Jaworski notes, this is often called the modal argument for substance dualism. It garners this name because it appeals directly to the possibility of a mind or person existing without a body. Possibility is, of course, a modal concept.

The argument as stated is valid, but its two premises are in need of support.


2. The Argument for Premise 1
The first premise makes a simple identity-based claim. As such, it is relatively easy to defend it by appealing to the axioms of identity.

Identity is a relation of sameness or equivalence between two objects, events or states of affairs. In other words, x is said to be identical to y, when the labels “x” and “y” can be used to refer to the same object, event or state of affairs. Thus, we can say that February 14th is identical to Valentine’s day because those labels refer to the same day; or that “Samuel Clemens” is identical to “Mark Twain” because both labels refer to the same person.

So if x and y are identical, they must be the very same object, event or state of affairs. In that case, x cannot exist without y. Why not? Because to say that x exists but y does not would be to deny the identity relationship that was originally presumed.

Applying this to the mind-body debate we can say the following. If, as some physicalists claim, the mind and body (or, at a minimum, a defined proportion of the brain) are identical, then one cannot exist without the other. And contrariwise, if the mind and body are not identical, as substance dualists claim, then one can exist without the other.

To lay this out more formally:

• (1.1) If x and y are identical, then x cannot exist without y; but if x and y are not identical, then x can exist without y (axioms of identity).

• (1.2) Therefore, if we are identical to our bodies, then we cannot exist without them; but if we are not identical to our bodies, we can exist without them.

• (1) Therefore, if we can exist without our bodies, then we cannot be bodies.

Jaworski notes that there is nothing to really dispute in this line of reasoning. So if there is a problem with the substance dualist argument, it is unlikely to be found through a critique of premise (1). Premise (2) must be where all the action is. And there are two arguments in its favour to be considered.


3. The Conceivability Argument for Premise 2
Premise (2) states that it is possible for the mind (for “us”) to exist without the body. But how can we say this with any confidence? One way would be to rely on a conceivability-possibility principle (a “CP” for short). A CP states, roughly, that if it is conceivable that x is true (or false) then it is also possible that x is true (or false). Since premise (2) relies on a claim about what is possible, a CP can then form the basis of the following type of argument in its favour:

• (2.1) If it is conceivable that x can exist without y, then it is possible that x can exist without y.

• (2.2) It is conceivable that we could exist without bodies (including brains).

• (2) Therefore, we can exist without bodies.

We won’t be evaluating this argument right now (we’ll leave that ‘til the next post), but we do need to ask what kind of examples can be adduced in support of (2.2). Jaworski identifies three:

(2.2.1) The Afterlife Example: Many people can conceive of themselves existing after they die (this is true even of those who don’t believe in the afterlife). But once they die, the physical body is destroyed. Therefore, in order to conceive of the afterlife these people must tacitly conceive of their existence without their bodies.

(2.2.2) The Annihilation Example : Imagine you are watching yourself sitting reading your computer screen through some security camera feed. Now imagine that the body you see sitting reading the computer is instantaneously annihilated and you are left staring at an empty chair. You are now conceiving of yourself existing without a body.

(2.2.3) The Cartesian Demon Example: Follow Descartes‘ sceptical procedure. Suppose that all your visual, auditory, olfactory and other sensory experiences are the product of a deception. By doing so, you’ll find that you can doubt the existence of the external world, including your own body, but that you cannot doubt your own existence. Thus, once again, you can conceive of your own existence in the absence of a body.

There are serious questions to be asked about these examples and whether they actually establish what they need to establish. We’ll leave those to one side for the time being and move on to consider the second argument in favour of premise (2).


4. The Essential Property Argument for Premise 2
Objects, events and states of affairs have properties. These properties come in two major forms: the essential and the accidental. An essential property is a property that an object, event or state of affairs cannot be without; an accidental property is one that it can be without.

To give an example, three-sidedness is a property that a triangle cannot be without. A triangle cannot be said to exist if it only has two sides. Therefore, three-sidedness is an essential property of a triangle. In contrast, “my favourite shape”-ness is an accidental property of a triangle. Triangles presumably existed before I was born and so did not require the property of being my favourite shape in order to exist.

The essential/accidental property distinction can be used as the basis of the following argument in favour of (2):

• (2.3) An object, event or state of affairs can exist without its accidental properties; it cannot exist without its essential properties.

• (2.4) An essential property of beings like us is that of thought or thinking; having a body is a strictly accidental property of beings like us.

• (2) Therefore, we can exist without bodies.

This argument owes its origins to the work of Descartes. The crucial premise here is (2.4). For it to be true, we need to have some reliable procedure for identifying what is an essential property and what is an accidental property; and we need that procedure to lead us to the claim stated in (2.4).

Jaworski points out two methods for identifying essential and accidental properties. One is the method proposed by empirical essentialists. They believe that the essential properties of an entity can only be identified after an exhaustive empirical study of that entity. The alternative method is that proposed by conceptual essentialists. They believe an a priori thought experiment can reveal what is essential and what is not.

Following the lead set by Descartes, the conceptual method is generally preferred by substance dualists. Jaworski describes the method as follows:

“According to Descartes, if we want to discern the essential properties of some object, A, we start by compiling a list of all the properties we take A to possess. Suppose, for instance, that P1, P2, P3...Pn is a list of those properties. We next take each property on the list, and consider whether it is possible for A to exist without it...[If] we cannot conceive of A existing without P1, then we can conclude that P1 is an essential property of A, that A needs P1 to exist. By following this procedure for every property on the list, Descartes thinks, we arrive at a list of A’s essential properties.” (pp. 42-43, with some emendations).

Following this method, we should find that having a body is a strictly accidental property of beings like us.

I know I said I’d hold off on criticisms and evaluations until future posts, but I can’t help but point out that this conceptual method seems to make the essential property argument either circular or indistinguishable from the conceivability-possibility argument. It would seem to do the former by making a claim about what it is possible for an entity to exist without dependent on a set of claims about what it is possible for an entity to exist without; and it would seem to do the latter by making what it is conceivable for an entity to exist without determinative of the essential properties of that entity.

To be fair, Jaworski is aware of these points, as is clear when he discusses the criticisms of the argument. I’ll look at these next time round. For now, I simply present a complete map of the argument for substance dualism. As always, click on the image to see a larger version.