DESCARTES AND THE ANIMAL MACHINE
Descartes's doctrine that animals are pure machines, while men are machines with minds, was in part a compromise between his scientific aims and his voluntaristic, Christian view of man. If biological phenomena could be included in the domain of his universal physics, then the boundary would no longer lie between inanimate and animate beings; physics would include all of nature except the mind of man. Harvey's discovery of the circulation of the blood encouraged Descartes to attempt a general mechanistic physiology in hydraulic terms. Descartes argued that most human motions do not depend on the mind and gave examples of physiological functions (such as digestion), reactions; (such as blinking, and feelings (such as passions) which occur independently of the will. In man, however, the mind could also direct the course of the fluid ("animal spirits") which controls movements.

However, to attribute minds to animals would threaten traditional religions beliefs, since the psychological concept of mind was conflated with the theological concept of soul. Descartes argued that it would be impious to imagine that animals have souls of the same order as men and that man has nothing more to hope for in the afterlife, than flies and ants have. Similarly, God could not allow sinless creatures to suffer; without souls, animals would not suffer, and man would be absolved from guilt for exploiting, killing and eating them. But he considered the most important reason for denying souls to animals to be their failure "to indicate either by voice or signs that which could be accounted for solely by thought and not by natural impulse" (letter to Henry More, February 1649). Thus, the use of language became the criterion of thought-"the true difference between man and beast." This argument has been accepted in much of the subsequent debate, and discussion has centered on the characteristics of a "true language."

It has often been suggested that Descartes was not consistent because occasionally he did ascribe mental functions to animals-sensation, imagination, passions, memory. Although some passages support this view, it seems clear that he attempted to maintain a rigid dualism by granting these functions to animals yet insisting that they were purely corporeal, while in man alone they had a mental counterpart; for instance, man had both corporeal and mental perceptions, and the dualism of mind and matter extended into his account of feelings. If attention is confined to the animal-machine hypothesis, it might therefore appear that there is little to choose between the Cartesian account and the views of his opponents, who did attribute mental functions to animals. The very extensive literature on the animal soul controversy lends support to this contention. However, the debate was not primarily about what animals could do but about the implications for man of various interpretations of their behavior. More generally, it concerned the adequacy of mechanistic explanation to account for biological and psychological phenomena.

Descartes excluded explanation by purpose (the "final causes" of the Aristotelian tradition) from physics and from biology. Yet mechanistic explanation was remarkably unsuccessful in accounting for biological phenomena without making some implicit or explicit appeal to concepts derived from mental intention or without postulating some intermediate substance or special vital force. The discontinuity in Cartesian metaphysics represented, then, a highly unstable compromise. The application of mechanism to animals and to the human body had considerable utility as an alternative to animistic explanation in physiology, but the demands of functional explanation in biology made it ultimately untenable. Science, the analogy between animals and men, and common sense called for a continuity which metaphysics and theology denied.
-------------------------------------------


I'm taking a chance by putting Descartes before the horse....