 These worries are to some extent alleviated by taking into account an important background idea in Wimsatt’s approach: Levels of organization and the entities that occupy them should be robust, meaning that they should be detectable, measurable, derivable, definable, and so on, in a variety of independent ways (1981 [2007: 63–4]; 1994 [2007: 210]). In other words, they should exhibit redundancy between independent means of accessing, detecting or defining them, and therefore should not depend on any single criterion or defining feature. Thus, if levels of organization are robust, the choice of criteria, or the problems of some individual criterion, will not be crucial. However, the extent to which there are such robust levels of organization in nature remains an open question.