This dissertation examines the potential relations between the objects and processes different sciences study. It examines the history, motivations, and analytical consequences of the thesis that individual sciences (such as physics, biology, and psychology) essentially pick out and describe different 'levels' of reality. It argues against the notion that there are different levels of reality that divide empirical domains of enquiry from one another. It proposes an alternate, more fluid, conception of scientific boundaries based upon patterns of causation and spatio-temporal contexts.