8.  Instance in Gold.

‘All gold is fixed,’ is a proposition whose truth we cannot be certain of, how universally soever it be believed.  For if, according to the useless imagination of the Schools, any one supposes the term gold to stand for a species of things set out by nature, by a real essence belonging to it, it is evident he knows not what particular substances are of that species; and so cannot with certainty affirm anything universally of gold.  But if he makes gold stand for a species determined by its nominal essence, let the nominal essence, for example, be the complex idea of a body of a certain yellow colour, malleable, fusible, and heavier than any other known;—­in this proper use of the word gold, there is no difficulty to know what is or is not gold.  But yet no other quality can with certainty be universally affirmed or denied of gold, but what hath a discoverable connexion or inconsistency with that nominal essence.  Fixedness, for example, having no necessary connexion that we can discover, with the colour, weight, or any other simple idea of our complex one, or with the whole combination together; it is impossible that we should certainly know the truth of this proposition, that all gold is fixed.

9.  No discoverable necessary connexion between nominal essence gold, and other simple ideas.

As there is no discoverable connexion between fixedness and the colour, weight, and other simple ideas of that nominal essence of gold; so, if we make our complex idea of gold, a body yellow, fusable, ductile, weighty, and fixed, we shall be at the same uncertainty concerning solubility in aqua regia, and for the same reason.  Since we can never, from consideration of the ideas themselves, with certainty affirm or deny of a body whose complex idea is made up of yellow, very weighty, ductile, fusible, and fixed, that it is soluble in aqua regia:  and so on of the rest of its qualities.  I would gladly meet with one general affirmation concerning any will, no doubt, be presently objected, Is not this an universal proposition, all gold is malleable?  To which I answer, It is a very complex idea the word gold stands for.  But then here is nothing affirmed of gold, but that that sound stands for an idea in which malleableness is contained:  and such a sort of truth and certainty as this it is, to say a centaur is four-footed.  But if malleableness make not a part of the specific essence the name of gold stands for, it is plain, all gold is malleable, is not a certain proposition.  Because, let the complex idea of gold be made up of whichsoever of its other qualities you please, malleableness will not appear to depend on that complex idea, nor follow from any simple one contained in it:  the connexion that malleableness has (if it has any) with those other qualities being only by the intervention of the real constitution of its insensible parts; which, since we know not, it is impossible we should perceive that connexion, unless we could discover that which ties them together.