5.  This more particularly concerns Substances.

The names of substances, then, whenever made to stand for species which are supposed to be constituted by real essences which we know not, are not capable to convey certainty to the understanding.  Of the truth general propositions made up of such terms we cannot be sure. [The reason whereof is plain:  for how can we be sure that this or that quality is in gold, when we know not what is or is not gold?  Since in this way of speaking, nothing is gold but what partakes of an essence, which we, not knowing, cannot know where it is or is not, and so cannot be sure that any parcel of matter in the world is or is not in this sense gold; being incurably ignorant whether it has of has not that which makes anything to be called gold; i. e. that real essence of gold whereof we have no idea at all.  This being as impossible for us to know as it is for a blind man to tell in what flower the colour of a pansy is or is not to be found, whilst he has no idea of the colour of a pansy at all.  Or if we could (which is impossible) certainly know where a real essence, which we know not, is, v.g. in what parcels of matter the real essence of gold is, yet could we not be sure that this or that quality could with truth be affirmed of gold; since it is impossible for us to know that this or that quality or idea has a necessary connexion with a real essence of which we have no idea at all, whatever species that supposed real essence may be imagined to constitute.]

6.

On the other side, the names of substances, when made use of as they should be, for the ideas men have in their minds, though they carry a clear and determinate signification with them, will not yet serve us to make many universal propositions of whose truth we can be certain.  Not because in this use of them we are uncertain what things are signified by them, but because the complex ideas they stand for are such combinations of simple ones as carry not with them any discoverable connexion or repugnancy, but with a very few other ideas.

7.

The complex ideas that our names of the species of substances properly stand for, are collections of such qualities as have been observed to co-exist in an unknown substratum, which we call substance; but what other qualities necessarily co-exist with such combinations, we cannot certainly know, unless we can discover their natural dependence; which, in their primary qualities, we can go but a very little way in; and in all their secondary qualities we can discover no connexion at all:  for the reasons mentioned, chap. iii.  Viz. 1.  Because we know not the real constitutions of substances, on which each secondary quality particularly depends. 2.  Did we know that, it would serve us only for experimental (not universal) knowledge; and reach with certainty no further than that bare instance:  because our understandings can discover no conceivable connexion between any secondary quality and any modification whatsoever of any of the primary ones.  And therefore there are very few general propositions to be made concerning substances, which can carry with them undoubted certainty.