PSYCHOLOGICAL SCIENCE 85 



acquired by direct sense-perception, when what we 

 know is the perception the sensation of green when 

 we look at grass, or the sensation of force when we 

 stop a moving ball. But it may also be derived 

 from memory, which makes us aware of what we 

 remember by the direct action of the mind. Thirdly, 

 another sort of knowledge by acquaintance is ac- 

 quired by introspection, in what is called self-con- 

 sciousness. When we see the grass, we may be 

 aware of our seeing the grass, aware of our own 

 mental state and mental action. Self -consciousness 

 is the source of all our knowledge of our own mental 

 life. 



The other division of knowledge by experience is 

 that kind which is acquired by description. Our 

 knowledge of the grass, as distinct from our know- 

 ledge of our sense-perceptions of it, is knowledge by 

 description. It involves the memory of a number 

 of descriptions of the qualities of the grass, either 

 discovered by ourselves or supplied to us by others 

 by the help of language. Our concept of grass 

 involves such properties as being green, of being good 

 food for cattle, of being made of growing vegetable 

 cells, themselves composed of molecules, atoms, or 

 electrons. We cannot be aware by acquaintance or 

 direct mental action of the grass, only of our sense- 

 perceptions ; we only know the grass, if we can be 

 said to know it at all, by consciously or unconsciously 

 putting together a number of descriptions of it. 



