 Our complex idea of gold might include its color, heaviness, solidity, etc., but each of these ideas can be set aside and considered all on its own as a simple idea. One simple idea, e.g., that of the yellowness of the lump of gold, is simple because it is a single, uniform appearance of that particular shade and not a collection of more basic appearances. The same goes for our idea of its hardness and heaviness. We can mentally separate the color from the hardness or heaviness and think of it all on its own.