METAPHYSICS on the complete organism Thus, ducks grow webbed feet so that they can swim Descartes rejected the use of teleological explanation in physics or biology Final causation, he maintained, implied in the agent a knowledge of the end to be pursued; but such knowledge could only exist in minds The explanation of every physical movement and activity must be mechanistic; that is, it must be given in terms of initial, not final, conditions, and those conditions must be stated in descriptive, not evaluative, terms Descartes offered no good argument for his contention, and his thesis ruled out straightforward gravitational attraction no less than the Aristotelian cosmic ballet Moreover, Descartes was wrong to think that teleological explanation must involve conscious purpose: whatever Aristotle may have thought about the heavenly bodies, he never believed that an earthworm, let alone a falling pebble, was in possession of a mind It was not Descartes, but Newton and Darwin, who dealt the serious blows to Aristotelian teleology, by undermining, in different ways, its two constituent elements Newtonian gravity, no less than Aristotelian motion, provides an explanation by reference to a terminus: gravity is a centripetal force, a force ‘by which bodies are drawn, or impelled, or in any way tend, towards a point as to a centre’ But Newton’s explanation is fundamentally different from Aristotle’s in that it involves no suggestion that it is in any way good for a body to arrive at the centre to which it tends Darwinian explanations in terms of natural selection, on the other hand, resemble Aristotle’s in demanding that the terminus of the process to be explained, or the complexity of the structure to be accounted for, shall be something that is beneficial to the relevant organism But unlike Aristotle, Darwin explains the processes and the structures, not in terms of a pull by the final state or perfected structure, but in terms of the pressure of the initial conditions of the system and its environment The red teeth and red claws involved in the struggle for existence were, of course, in pursuit of a good, namely the survival of the individual organism to which they belonged; but they were not in pursuit of the ultimate good that is to be explained by selection, namely, the survival of the fittest species It is thus that the emergence of particular species in the course of evolution could be explained not only without appeal to a conscious designer, but without evoking teleology at all 176