The course of nature, although it is supposed to be perfectly regular, yet it does not appear to us so, and many events are unsafe and many more disappoint our expectations.
When we look around for the outer objects and consider the action of the causes, we are never able to discover any necessary force or connection, any quality that combines the effect to the cause, making it one of the infallible consequence of the other, considering only one case.
We just find that in reality the effect comes after the cause. The momentum of a billiard ball is followed by the second ball. This is all that appears to our outer senses.
We can see that this influence, just like all other natural events, can only be known through experience and can never be predicted on the basis of any energy or force that would bind it to effect and make the last infallible consequence of the first.
The movement of our body follows the command of our will and we are aware of it every moment. But as to the means by which this operation is performed or the energy through which the will performs such an extraordinary operation, we are so far away from being directly aware that they will always escape our most thorough research.
So, first of all, is there any more mysterious principle in the whole nature than the union of soul and body by which the supposed spiritual substance has such an influence on the material one that the most subtle thought is capable of moving the grossest matter ?
If we had the power that by a mysterious desire to move the mountains or control the planets in their orbits, such an immense power would be no more extraordinary and far beyond our power of understanding.
from David Hume, An Enquiry Concerning Human Understanding
EQUINOX 