This article develops a quality notion that is complementary to the system notion. As a major consequence, it becomes clear why quality can be measured only to a certain extend based on the issues of validity and incompleteness. First, there is an inherent conflict between the applicability and validity of quality measures and second, quality considerations almost always refer to high-dimensional spaces with only sparse knowledge also known as "curse of dimensionality". The resulting gap of knowledge has to be filled by experienced based heuristics. To deal with the curse of dimensionality, the heuristics of categorizing qualities into strategic and necessary is proposed. Strategic qualities provide contrast, while necessary qualities rather diminish contrast. In an economic context the presence of strategic qualities motivate a buy-decision and the absence of necessary qualities motivate a don't-buy-decision.