 In his discussion of names of substances and in the contrast between names of substances and names of modes, a number of interesting features of Locke’s views about language and knowledge emerge. Physical substances are atoms and things made up of atoms. But we have no experience of the atomic structure of horses and tables. We know horses and tables mainly by secondary qualities such as color, taste and smell and so on and primary qualities such as shape, motion and extension. So, since the real essence (the atomic constitution) of a horse is unknown to us, our word ‘horse’ cannot get its meaning from that real essence. What the general word signifies is the complex of ideas we have decided are parts of the idea of that sort of thing. These ideas we get from experience. Locke calls such a general idea that picks out a sort, the nominal essence of that sort.