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Descartes worked out his own detailed theories about the physical operation of the material world in Le Monde (The World), but uncertainty about ecclesiastical reactions prevented him from publishing it. The final sections of the Discourse, however, include several significant hints about the positions he was prepared to defend. Their explanations of the activities of living organisms make the mechanistic implications of the Cartesian view more evident.
Since, as everyone acknowledges, non-human animals do not have souls, Descartes concluded that animals must be merely complex machines.
Since they lack any immaterial thinking substance, animals cannot think, and all of the movements of their bodies can, in principle, be explained in purely mechanical terms.
(Descartes himself incorrectly supposed that the nervous system functions as a complex hydraulic machine.)
But since the structure of the human body and the behavior of human beings are similar to the structure and behavior of some animals, it is obvious that many human actions can also be given a mechanistic explanation.
La Mettrie later followed this line of reasoning to its ultimate conclusion, supposing human beings to be nothing more than Cartesian machines.
The philosophy of Descartes won ready acceptance in the second half of the seventeenth century, expecially in France and Holland. Although few of his followers, known collectively as Cartesians, employed his methods, they showed great diligence and ingenuity in their efforts to explain, defend, and advance his central doctrines.
In the physical sciences, for example, Cavendish, Rohault, and Régis were happy to abandon all efforts to employ final causes in their pursuit of mechanistic accounts of physical phenomena and animal behavior. On this basis, however, such philosophers were able to progress beyond a simple affirmation of the mysterious reality of mind-body interaction.
Metaphysicians like Cordemoy and Geulincx fared little better in their efforts to deal with this crucial problem with dualism. If there is no genuine causal interaction between independent substances, we seem driven to suppose that the actions of mind and body are merely parallel or divinely synchronized.
Not everyone was entirely satisfied by the epistemological foundations of the Cartesian scheme, either.
Critics like Arnauld,
Foucher drew attention to the inherent difficulty of explaining in
representationalist terms how our ideas of things can be known to resemble the things themselves and the implausibility of reliance upon innate ideas.
Conway went even further, rejecting the dualistic foundations of Descartes's substance-ontology along with his approach to human knowledge.
One seventeenth-century thinker of greater independent significance was Blaise Pascal, with his unusual blend of religious piety, scientific curiosity, and mathematical genius. Led by his deep religious feelings to participate fully in the pietistic Jansenism of the Port-Royal community, Pascal maintained that formal reasoning about god can never provide an adequate substitute for genuine personal concern for the faith: "The heart has its reasons that reason cannot know."
Pascal's mathematical acumen was no less remarkable than that of Descartes; his work anticipated the development of game theory and the modern methods of calculating
In fact, his famous "Wager" applies these mathematical techniques to the prudence of religious conviction in the absence of adequate evidence: since the consequences of believing are
infinitely beneficial if there is a god and only slightly inconvenient if there is not, while the outcome of atheism is only somewhat more pleasant if there is no god and eternally costly if there is, the
expected value of theism is much greater than that of atheism, and it is reasonable to stake one's life on the possibility that god does exist.
The most original and influential philosopher of the Cartesian tradition was Nicolas Malebranche. Noting the steady progress of efforts to provide mechanistic accounts of the behavior of the human body, Malebranche concluded that the mind and body are not only substantially distinct but causally independent of each other. The appearance of genuine interaction arises from what is in fact merely the perfect parallelism of events in the mental and physical realms.
According to Malebranche, then, our ideas of bodies do not result from any causal influence that physical objects have on our senses; rather, they are produced in our minds directly by god. Thus, he supposed, in sense perception what literally happens is that we "see all things in god." Similarly, our wills have no causal influence on the material world, but god provides for the coordination of our volitions with the movement of bodies. In general, since there is no causal interaction, it is the power of god alone that secures a perpetual, happy coincidence of the states and operations of minds and bodies.
Since only god's activity is efficacious in either mental or physical things, apparent causes in either realm are merely the occasions for the appearance of their supposed effects in the other.
Thus, the views of Malebranche are often referred to collectively as occasionalism.
Although the entire theory found few enthusiastic adherents, Malebranche's analysis of the regularities exhibited in nature by causally independent beings and events was greatly influential on later philosophers, including
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