“For indeed, no one has yet determined what the body can do, that is, experience has not yet taught anyone wha the body can do from the laws of Nature alone, insofar as Nature is only considered to be corporeal, and what the body can do only if it is determined by the mind. For no one has yet come to know the structure of the body so accurately that he could explain all its functions – not to mention that many things are observed in the lower animals which far surpass human ingenuity, and that sleepwalkers do a great many things in their sleep which they would not dare to awake. This shows well enough that the body itself, simply from the laws of its own nature, can do many things which its mind wonders at.
Again, no one knows how, or by what means, the mind moves the body, nor how many degrees of motion it can give the body, nor with what speed it can move it. So it follows that when men say that this or that action of the body arises from the mind, which has dominion over the body, they do not know what they are saying, and they do nothing but confess, in fine-sounding words, that they are ignorant of the true cause of that action, and that they wonder at it.”
P2, part III. Of the Affects, Ethics, 1677