Descartes was a rationalist.  This means that he believed some knowledge ultimately comes from reason, pure thought.  Recall that he believed all knowledge was based on the basic idea that God is not a deceiver, and thus whatever he believes “clearly and distinctly” must be true.


On the other side of this issue, stands the empiricists.  They believed that all knowledge ultimately comes from experience.  The main proponents of this theory were Locke and Hume (note that Berkeley was an empiricist).  They believed that everything in our minds came from the senses and that if something was not derived from the senses, it could not be a real idea.  This is what lead Hume to conclude that things like “causation,” “liberty,” and “self” were not really ideas, but meaningless words we throw around.


Believing empiricism was true, many of the early psychologists argued that the only way to conduct a science of the mind was through observable behavior (since other people’s thoughts and beliefs are not observable).  The most famous psychological behaviorist was B.F. Skinner, who felt that if there were “mental states,” they had no effect on behavior.  Thus, the focus should be on behavior, not mental states.


Many philosophers saw what Skinner was doing, and believing in empiricism, developed that idea that mental states were nothing but our physical behavior.  This was their reasoning:


  1. Something is not a real idea unless it is derived from sense experience
  2. Thus, a sentence is meaningful only if it contains ideas which were derived from sense experience. (this is known as logical empiricism).  So, if we cannot tell whether a         sentence is true by looking to the sensible world, then it is meaningless.


  1. Because of this, these theorists believed that the meaning of a sentence just is its method of verification.  For example, to understand what the sentence, “The cat is on the           mat” means just is to understand how to verify it.  (this is the verifiability theory of           meaning)


  1. So, now consider how one determines whether another person is thirsty, or in pain, or in love.  What you do is to look at their behavior.  Since the method of verification is an    examination of external behavior, it follows that all we mean by “thirsty,” “pain,” or       “love,” is the external behavior, period.


Logical behaviorism is a better theory than Cartesian Dualism in the sense that it is simpler, it postulates fewer entities in the world.  It has also been more fruitful and more conservative than Cartesian dualism (e.g., it solves the problem of other minds, allows predictions and fits with existing scientific theories).



  1. This theory is false if it is possible for a person to “be in pain,” and not exhibit external behaviors consistent with “being in pain.” Also, the theory is false if it is possible for a person to exhibit the behavior of “being in pain” without “being in pain.” (Consider thought experiments p.68)…..since mental states are “feelings”–qualia–then these counterexamples seem possible.


  1. The verifiability theory of meaning is flawed (step 3 above). The easiest way to see this is to ask, “how can we verify if the verifiability theory of meaning is true?” There is no way, hence by the theory itself, it is meaningless.


Identity Theory


The example of Phineas Gage (1848) was one of the first recorded cases where a change in the physiology of the brain lead to a marked change in one’s personality (mind).  Because of this case, and many others like it, people started to believe that there was a very close link between the physiology of the brain and the mind, or our mental states.  The doctrine that our mental states just are brain states is known as the identity theory.  Like logical behaviorism, identity theory does not assume the existence of any “mental stuff” (so it is a materialist theory).  And like logical behaviorism, it seems to fit well with existing scientific theory and to be able to explain many things which the Cartesian dualists cannot (like how the “mind” and the brain interacts).



  1. If true, then whatever is true of mental states is true of brain states. As the book points out, each will not necessarily share the same subjective properties (what someone thinks about it), but they must share all objective properties.

For example, you may not believe that Samuel Clemens wrote Huckleberry Finn, but              you do believe that Mark Twain wrote Huckleberry Finn.  The fact that Samuel Clemens            and Mark Twain do not share these subjective properties does not mean they are not       identical (they are).  To be identical they must share all objective properties (size,                 location, color).

And since there is some objective property of brain states that mental states do not have,  namely that of “being known through empirical investigation,” the identity theory has a problem.  Thomas Nagel argues that we can never know what it is like to be some other conscious thing (he uses a bat).


  1. If the identity theory were true, then it would be impossible for creatures without a brain to have mental states. However, this does seem logically possible, maybe even physically possible (consider the thought experiment; J. Searle’s “Brain Replacement”).