Mind/Body Problem


On pages 148-155 of Naming and Necessity, Saul Kripke concludes his book by setting forth an argument against type-type identity theory (the view that that any given mental state is identical to a brain state). Kripke argues that a mental state (pain) cannot be identical to a brain state (C-fiber stimulation) because if the aforementioned states are identical, then they must be so necessarily; and since we can conceive of one state existing without the other, they cannot be identical. Below, I shall set forth a general argument (with explanatory comments) which demonstrates the basis of Kripke’s reasoning and then cite Krikpe’s example from the text (also with explanatory comments).

1. If X is a rigid designator and Y is a rigid designator, and if the statement “X is identical to Y” is true, then the statement “X is identical to Y” is necessarily true.

• An example of the above is “Mark Twain is identical to Samuel Clemens”. Since both of these names are rigid designators, then “Mark Twain is identical to Samuel Clemens” is true in every possible world in which these rigid designators pick out an individual. On the other hand, the statement “Mark Twain is identical to the author of The Adventures of Tom Sawyer” is composed of the rigid designator “Mark Twain” and the non-rigid designator “the author of The Adventures of Tom Sawyer”. While these two designators may be identical in the actual world, it is possible that someone else could have written The Adventures of Tom Sawyer. Thus, the latter example of identity is not necessary.

2. X is a rigid designator.

3. Y is a rigid designator.

4. However, “X is identical to Y” is not necessarily true.

5. Therefore, “X is identical to Y” is not true.

Below is a reconstruction of Kripke’s specific example involving “pain” and “C-fiber stimulation”:

1. If “pain” is a rigid designator and “C-fiber stimulation” is a rigid designator, then the statement “pain is identical to C-fiber stimulation” is necessarily true.

• Kripke anticipates that some people will object that the identity statement above cannot be necessary since it is known a posteriori. However, Kripke points out that the statement “pain is identical to C-fiber stimulation” is analogous to other necessary identity statements that are known a posteriori such as “water is identical to H2O” or “heat is identical to molecular motion”. Besides this, Kripke demonstrates early on (pp. 35-37) that some necessary truths are known a posteriori (like certain mathematical proofs that require a great deal of calculation).

2. “Pain” is a rigid designator.

• Once again Kripke anticipates objections to the above premise. He asserts that “pain” is a rigid designator because it picks out what it refers to by an essential property (the sensation of painfulness). It is impossible to conceive of pain existing without the property of “feeling painful”.

3. “C-fiber stimulation” is a rigid designator.

4. However, “pain is identical to C-fiber stimulation” is not necessarily true.

• It is possible to conceive of a world in which one can exist without the other.

5. Therefore, “pain is identical to C-fiber stimulation” is not true.

I must confess that the above argument appears to be a powerful refutation of identity theory. It seems to me that the only way out of the conclusion is to either deny Krikpe’s semantics or argue convincingly that either “pain” or “C-fiber stimulation” or both are not rigid designators.

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Rene DescartesThere can be no doubt that modern Western thought owes an incredible debt to the work of the French philosopher, Rene Descartes.  His Meditations on First Philosophy revolutionized the method and scope of rational inquiry and has had lasting repercussions into the present day.  Within the Meditations Descartes employs a strict axiomatic method in search of a secure foundation for knowledge; a foundation which he eventually discovers in his famous statement, cogito ergo sum: “I think therefore I am”.  An interesting byproduct of this discovery is Descartes’ consequent assertion that it is easier to know that one has a mind then that one has a body.  Indeed, the title of the second Meditation is, “Of the Nature of the Mind; And That It Is More Easily Known than the Body”.  But is this statement implied by Descartes’ cogito?  I believe that it is not and that Descartes’ reasoning is fallacious in regards to his epistemological distinction between the mind and the body.           

Before investigating the above claim, it is important to review the process of reasoning that led to its assertion.  Descartes begins the Meditations with radical doubt.  Indeed his stated purpose is to “apply [him]self earnestly and freely to the general overthrow of all [his] former opinions.” His criterion for knowledge is strict foundationalism: he will only accept propositions that cannot be doubted or propositions that are built upon other indubitable propositions.  He is quite literally seeking to demolish the structure of his beliefs so that he can reconstruct them upon a certain foundation.  The epistemological wrecking balls that he employs are various arguments that can be traced back to the ancient skeptics.  At the end of the day – after meditating upon insanity, dreams, and evil demons – Descartes has effectively stripped himself of certainty concerning his former beliefs.

A new day provides the second Meditation and the famous discovery of the cogito.

So that it must, in fine, be maintained, all things being maturely and carefully considered, that this proposition I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.

There has been much debate over what Descartes was exactly trying to communicate by this statement.  Some have insisted that Descartes is presenting an argument here, while others believe that he is simply stating a self-evident truth which he has discovered.  While this debate is both interesting and important, it really makes no difference in regards to this post.  Suffice it to say that Descartes (whether in formal argument or not) is asserting that it is a necessary truth that whenever he thinks, he exists.  This is an impressive feat of reasoning to be sure, but his assumption that the mind is more easily known than the body does not follow from it.

            Descartes’ claim can be expressed in the following argument:

  • 1. I cannot doubt the existence of my mind.
  • 2. I can doubt the existence of my body.
  • 3. Therefore, the mind is more easily known than the body.

The first premise follows from the cogito.  Descartes states that he cannot doubt his own existence because, necessarily, whenever he doubts he is thinking, and whenever he is thinking he exists.  Therefore he exists as a mind, or a “thinking thing”.  In the first Meditation, however, he has already established that he cannot be certain of the existence of his body.  Therefore the knowledge of his mind is more certain than the knowledge of his body. 

            It is important to point out a major assumption that Descartes is making here: he assumes that the mind and body are distinct substances.  It appears that he bases this distinction upon a principle that was later articulated by the German philosopher Gottfried Leibniz.  This principle, known as Leibniz’s Law of the Indiscernibility of Identicals can be stated thus:

Necessarily, for any X and Y, if X is identical with Y, then for any property, P, X has P iff Y has P.

It follows then from the above principle that, if the mind and body possess different properties, they must be distinct substances.  Descartes appears to hold to something like the following argument:

  • 1. My body possesses the property of being doubted by me.
  • 2. My mind does not possess the property of being doubted by me.
  • 3. Therefore, according to the Indiscernibility of Identicals, my body is distinct from my mind.

It is only after he assumes the distinction above that Descartes can then go on to argue that the mind is more easily known than the body.  However, this argument is based upon the fallacious idea that dubitability implies distinction. 

            Consider the following argument:

  • 1. Samuel Clemens possesses the property of being doubted by me that he is the author of A Connecticut Yankee in King Arthur’s Court.
  • 2. Mark Twain does not possess the property of being doubted by me that he is the author of A Connecticut Yankee in King Arthur’s Court.
  • 3. Therefore, according to the Indiscernibility of Identicals, Samuel Clemens is not the same person as Mark Twain.

Of course this argument is clearly absurd since we know that the names “Samuel Clemens” and “Mark Twain” do indeed refer to the same person.  It is this same reasoning, however, that Descartes employs in distinguishing the body from the mind.  Just because he doubts the existence of his body and is certain of the existence of his mind, it does not follow that they are two different substances.

            It appears, therefore, that Descartes was wrong in asserting that the mind is more easily known than the body based upon the cogito.  What he was perceiving as his “body” and as his “mind” may well have been the same substance with two different names.  This is not to imply that the body and mind are the same, however, just that one cannot make this distinction based upon Descartes’ assumption.  And if the mind and body are indeed the same substance, then it makes no sense to say that one is more easily perceived than the other.