28.  Secondly, Another cause, Want of a discoverable Connexion between Ideas we have.

Secondly, What a small part of the substantial beings that are in the universe the want of ideas leaves open to our knowledge, we have seen.  In the next place, another cause of ignorance, of no less moment, is a want of a discoverable connection between those ideas we have.  For wherever we want that, we are utterly incapable of universal and certain knowledge; and are, in the former case, left only to observation and experiment:  which, how narrow and confined it is, how far from general knowledge we need not be told.  I shall give some few instances of this cause of our ignorance, and so leave it.  It is evident that the bulk, figure, and motion of several bodies about us produce in us several sensations, as of colours, sounds, tastes, smells, pleasure, and pain, &c.  These mechanical affections of bodies having no affinity at all with those ideas they produce in us, (there being no conceivable connexion between any impulse of any sort of body and any perception of a colour or smell which we find in our minds,) we can have no distinct knowledge of such operations beyond our experience; and can reason no otherwise about them, than as effects produced by the appointment of an infinitely Wise Agent, which perfectly surpass our comprehensions.  As the ideas of sensible secondary qualities which we have in our minds, can by us be no way deduced from bodily causes, nor any correspondence or connexion be found between them and those primary qualities which (experience shows us) produce them in us; so, on the other side, the operation of our minds upon our bodies is as inconceivable.  How any thought should produce a motion in body is as remote from the nature of our ideas, as how any body should produce any thought in the mind.  That it is so, if experience did not convince us, the consideration of the things themselves would never be able in the least to discover to us.  These, and the like, though they have a constant and regular connexion in the ordinary course of things; yet that connexion being not discoverable in the ideas themselves, which appearing to have no necessary dependence one on another, we can attribute their connexion to nothing else but the arbitrary determination of that All-wise Agent who has made them to be, and to operate as they do, in a way wholly above our weak understandings to conceive.

29.  Instances

In some of our ideas there are certain relations, habitudes, and connexions, so visibly included in the nature of the ideas themselves, that we cannot conceive them separable from them by any power whatsoever.  And in these only we are capable of certain and universal knowledge.  Thus the idea of a right-lined triangle necessarily carries with it an equality of its angles to two right ones.  Nor can we conceive this relation, this connexion of these two ideas, to be possibly mutable, or to depend on any arbitrary power, which