Grounding Explanations

Consider some facts: water contains hydrogen, my colleague’s cat is alive, diamond is harder than granite, I prefer oatmeal to brussels sprouts, Beijing has over 14 million inhabitants, interest rates are low. These chemical, biological, geological, psychological, sociological, and economic facts all appear to rest on further facts. Facts involving cities, e.g., the fact that Beijing has over 14 million inhabitants, are not rock-bottom: city facts are determined by, dependent upon, and derived from facts about where people live, how they act, and what their attitudes are. These facts might on occasion be hard to discover or state, given the number of people involved and the trouble we have saying exactly which actions and attitudes do the relevant work. But we shouldn’t let the difficulty of the details distract us from the original, compelling idea: city facts rest on other facts, including facts about human beings; facts about human beings rest on other facts, including facts about organs, cells, and genes; these facts in turn rest on chemical facts; and so it goes, at least for a while. Reality comes in layers. We often disagree about what there is at the bottom, or even if there is a bottom. But we agree that higher up we find facts involving a diverse array of entities, including chemical, biological, geological, psychological, sociological, and economic entities; molecules, human beings, diamonds, mental states, cities, and interest rates all occupy higher layers. The nature and existence of the entities in the higher layers are determined by, dependent upon, and derived from the more fundamental facts and entities we find lower down. So, it seems, there is a layered structure of facts and the entities those facts involve. How is this intuitive talk of layered structure to be understood? One option is to cash out layering in terms of reduction: the upper layers – the chemical, biological, geological, etc.– are all reducible to lower layers. Another option is to rely instead on supervenience: upper layers asymmetrically supervene on lower