Hierarchies and Levels of Reality

Talk of levels or layers of reality is ubiquitous in science and in philosophy. It is widely assumed, for instance, that physical, chemical, biological, and mental phenomena can be ordered in a hierarchy of levels, and that what happens at the so-called micro level determines the goings on at the macro level. Questions about both reduction and emergence presuppose some structure of levels; without this, the very question of whether higher levels reduce to more basic ones could not be asked. Often intuitions seem sufficient to establish the ordering and the nature of the levels. But, given the central importance of this picture, more should be said about how we identify levels, what populates them (objects, properties, facts, …), what principles govern the hierarchy, and how widely any particular account of levels is applicable.1 In this paper we address these questions, taking our starting point within (classical) physics. We examine the features of the hierarchies, ordering principles, and population of levels in physics and argue that the structure of levels suggested by physics is quite different from that assumed in widely assumed philosophical schemes, in particular Oppenheim and Putnam’s (1958) account of levels of reality