countable and uncountable distinctions

0Hi,02br
02br
00I think someone has said that at least in thery, there is a default for very word, thus, making it possible for every word in theory to be uncountable if used in terms of its quality. I am not trying to dispute the acceptabililty of it but rather want to ask you why the word 'strength' is listed as qualities and 'weakness' is not. To me, they both give out the similar, if not the same, lexical nuances. Why for 'weakness' we can add 'a' to indicate a single, individual weakness; whereas, for 'strength' we don't seem to use 'a' before it very often, if none ever? 0-