In 'A Materiali,;t Theory of the Mind', D. M. Armstrong suggests that we should construe desires as states apt to cause goal-seeking or purposive behavior.1 Since Armstrong's account of purposive behavior is a modified version of Braithwaite's, 2 what this comes to is that a desire is essentially a state causing an organism to adjust its behavior in response to changes in its environment, and so to achieve a given result (its 'goal') under a variety of different actual or possible conditions. In this paper, I want to ask whether this important analysis can adequately account for the existence of desires that are impossible to satisfy.
Celtainly, the analysis has no problem with desires that are merely not satisfied: a person will have such a desire for 0 on Armstrong's account whenever he is in a state that has not (yet) actually caused him to obtain 0, but wouM cause him to do so under an appropriate number of alternative conditions. But what could it be for someone to have a desire that is not merely unsatisfied, but is impossible to satisfy -a desire to swim the Atlantic, say, or a desire to square the circle? Since it is physically impossible for a normal man to swim the Atlantic, there are no possible conditions under which such a man would swim the Atlantic, and so a fortiori no states that would cause him to do this under any possible conditions; and, similarly, since it is logically impossible for anyone to square the circle, there are no possible conditions under which anyone would square the circle, and so again a fortiori no states that would cause anyone to do so under any possible circumstances. A consistent application of Armstrong's analysis of desire therefore seems to lead us to the absurd conclusion that no normal man can desire to swim the Atlantic, and that no one at all can desire to square the circle.
Armstrong acknowledges that there is a difficulty here, but thinks it can be avoided. He writes that