This sceptical doubt, both with respect to reason and the senses, is a malady, which can never be radically cured, but must return upon us every moment, however we may chace it away, and sometimes may seem entirely free from it.  It is impossible upon any system to defend either our understanding or senses; and we but expose them farther when we endeavour to justify them in that manner.  As the sceptical doubt arises naturally from a profound and intense reflection on those subjects, it always encreases, the farther we carry our reflections, whether in opposition or conformity to it.  Carelessness and in-attention alone can afford us any remedy.  For this reason I rely entirely upon them; and take it for granted, whatever may be the reader’s opinion at this present moment, that an hour hence he will be persuaded there is both an external and internal world; and going upon that supposition, I intend to examine some general systems both ancient and modern, which have been proposed of both, before I proceed to a more particular enquiry concerning our impressions.  This will not, perhaps, in the end be found foreign to our present purpose.

SECT.  III.  OF THE ANTIENT PHILOSOPHY.

Several moralists have recommended it as an excellent method of becoming acquainted with our own hearts, and knowing our progress in virtue, to recollect our dreams in a morning, and examine them with the same rigour, that we would our most serious and most deliberate actions.  Our character is the same throughout, say they, and appears best where artifice, fear, and policy have no place, and men can neither be hypocrites with themselves nor others.  The generosity, or baseness of our temper, our meekness or cruelty, our courage or pusilanimity, influence the fictions of the imagination with the most unbounded liberty, and discover themselves in the most glaring colours.  In like manner, I am persuaded, there might be several useful discoveries made from a criticism of the fictions of the antient philosophy, concerning substances, and substantial form, and accidents, and occult qualities; which, however unreasonable and capricious, have a very intimate connexion with the principles of human nature.

It is confest by the most judicious philosophers, that our ideas of bodies are nothing but collections formed by the mind of the ideas of the several distinct sensible qualities, of which objects are composed, and which we find to have a constant union with each other.  But however these qualities may in themselves be entirely distinct, it is certain we commonly regard the compound, which they form, as one thing, and as continuing the same under very considerable alterations.  The acknowledged composition is evidently contrary to this supposed simplicity, and the variation to the identity.  It may, therefore, be worth while to consider the causes, which make us almost universally fall into such evident contradictions, as well as the means by which we endeavour to conceal them.