This proneness to require the appearance of some necessary and natural connection between the cause and its effect, i.e. some reason _per se_ why the one should produce the other, has infected most theories of causation. But the selection of the particular agency which is to make the connection between the physical antecedent and its consequent seem _conceivable_, has perpetually varied, since it depends on a person's special habits of thought. Thus, the Greeks, Thales, Anaximenes, and Pythagoras, thought respectively that water, air, or number is such an agency explaining the production of physical effects. Many moderns, again, have been unable to _conceive_ the production of effects by volition itself, without some intervening agency to connect it with them. This medium, Leibnitz thought, was some _per se_ efficient physical antecedent; while the Cartesians imagined for the purpose the theory of Occasional Causes, that is, supposed that God, not _quâ_ mind, or _quâ_ volition, but _quâ_ omnipotent, intervenes to connect the volition and the motion: so far is the mind from being forced to think the action of mind on matter more _natural_ than that of matter on matter. Those who believe volition to be an efficient cause are guilty of exactly the same error as the Greeks, or Leibnitz or Descartes; that is, of requiring an _explanation_ of physical sequences by something [Greek: aneu hou to aition ouk an pot' eiê aition]. But they are guilty of another error also, in inferring that volition, even if it is an _efficient_ cause of so peculiar a phenomenon as nervous action, must therefore be the efficient cause of all other phenomena, though having scarcely a single circumstance in common with them. CHAPTER VI. THE COMPOSITION OF CAUSES.