6. Of their necessary co-existence in substances.
Thirdly, The third sort of agreement or disagreement to be found
in our ideas, which the perception of the mind is employed about, is co-existence or non-co-existence in the same
subject; and this belongs particularly to substances. Thus when we pronounce concerning gold, that it is fixed, our
knowledge of this truth amounts to no more but this, that fixedness, or a power to remain in the fire unconsumed,
is an idea that always accompanies and is joined with that particular sort of yellowness, weight, fusibility,
malleableness, and solubility in aqua regia, which make our complex idea signified by the word gold,
