substances vs. expressions 

I reverted the change made by  Boredzo, reinserting the passange "It is often errouneously thought that.." (in fact the word "erroneously" was not part of the original edit) Removing it substantially changed the meaning of the paragraph, and made it inconsistent.   The point is that mass nouns *cannot* be defined in terms of what they refer to, but must rather be defined with reference to their grammatical properties, such as co-occurrence restrictions.