Locke, in his 'An Essay concerning human understanding', says that secondary qualities - all that we sense such as color, sound, and so on - are actually from the primary qualities of insensible parts of the body. And when he says about the Essence, he means by 'Real Essence', the real internal constitution. From his great dependence on Boyle's scientific thoughts, I can't help thinking it same with the Primary quality. Can anyone tell the difference between them? - June .