These we learn by experience, and so obtain a sort of foresight which enables us to regulate our actions for the benefit of life.  In general, to obtain such or such ends such or such means are conducive; and all this we know, not by discovering any necessary connection between our ideas, but only by the observation of the laws of Nature.

And yet this constant uniform working, which so evidently displays the goodness and wisdom of that governing spirit whose will constitutes the laws of Nature, is so far from leading our thoughts to Him that it rather sends them wandering after second causes.  For when we perceive certain ideas of sense constantly followed by other ideas, and we know that it is not of our own doing, we forthwith attribute power and agency to the ideas themselves, and make one the cause of another, than which nothing can be more absurd.

II.—­THE ROOTS OF SCEPTICISM

Several difficult and obscure questions, on which abundance of speculation hath been thrown away, are by our own principles entirely banished from philosophy.  “Whether corporeal substance can think,” “whether matter be infinitely divisible,” “how matter operates on spirit”—­these and the like inquiries have given infinfte amusement to philosophers in all ages.  But since they depend on the existence of matter, they have no longer any place in our principles.  It follows, also, that human knowledge may be reduced to two heads—­knowledge of ideas, and knowledge of spirits.  Our knowledge of the former hath been much obscured and confounded, and we have been led into very dangerous errors, by supposing a twofold existence of the objects of sense, the one “intelligible,” or in the mind, the other “real,” and without the mind; whereby unthinking things are thought to have a natural subsistence of their own, distinct from being perceived by spirits.

This is the very root of scepticism; for so long as men thought that real things subsisted without the mind, and that their knowledge was only so far “real” as it was conformable to “real things,” they could not be certain that they had any real knowledge at all.

So long as we attribute a real existence to unthinking things, distinct from their being perceived, it is not only impossible for us to know the nature of any real unthinking being, but it is impossible for us even to know that it exists.  Hence it is that we see philosophers distrust their senses, and doubt of the existence of heaven and earth, of everything they see or feel.  But all this doubtfulness, which so bewilders and confounds the mind, vanishes if we annex a meaning to our words and do not amuse ourselves with the terms “absolute,” “external,” “exist,” and such like, signifying we know not what.  I can as well doubt of my own being as of the being of those things which I perceive by sense; the very existence of unthinking beings consists in their being perceived.