…a quality is merely something that might be realized, while an occurrence is something that actually takes place. [—] A quality […] has no individuality. Two qualities are different only so far as they are unlike. Individuality is an aggressive unity, arising from an absolute refusal to be in any degree responsible for anything else. This a quality cannot have since it is too utterly irrespective of anything else even to deny it.
http://www.commens.org/dictionary/entry/quote-topical-geometry-general-t-2