 O&amp;P’s second use of levels concerns the justificatory role the concept plays for their framework. This role is anticipated by O&amp;P’s ideas of microreduction and potential microreducer, and requires positing ‘levels’ as existing independent of the other elements of their framework. More precisely: The O&amp;P account of reduction essentially assumes the existence of certain empirical facts captured by potential microreducers, which are then accounted for, i.e., justified, by postulating the existence of ‘levels’. O&amp;P considered this dual epistemic-ontological importance of ‘levels’ to be a significant contribution provided by their layer-cake account apart from its role in explicating microreduction. Speaking to the epistemic ordering of the sciences, they say: