In a previous post, I looked at the idea of grounding graphs as global entities. But I think there is a more natural way of looking at them. There are two main views about grounding. On the truthmaker view, true propositions are grounded in entities that make them true. On the propositional view, true propositions are grounded in other true propositions. But I think a more natural approach is to say that propositions are grounded in graphs.
A candidate grounding graph for a proposition p is a directed graph G satisfying the following properties:
- all the vertices of G are true propositions
- p is a vertex of G
- all the vertices of G other than p are ancestors of p
- p is not the only vertex of G.
Define a proposition as fundamental provided that it is true but has no grounding graph. Say that a grounding graph for p is a candidate grounding graph for p that in fact grounds p. A vertex is initial provided that it has no ancestors and is final provided it has no children. A candidate grounding graph has exactly one final vertex. Say that G* extends G provided that (a) G* has the same final vertex as G and (b) every vertex of G that has a parent in G has exactly the same parents in G* as in G. The following are important properties of grounding graphs: Say that a graph where every initial vertex is fundamental is a fundamental graph.
- Acyclicity: Every grounding graph is acyclic.
- Extensibility: If G is a grounding graph for p, then there is a fundamental extension of G that is also a grounding graph for p.
- Adjoining: If G1 is a grounding graph for p, and G2 is a grounding graph for some initial vertex q of G1 such that G2 has a fundamental extension whose only vertex in common with G1 is q, then the graph whose vertex collection is the union of the vertex collections of G1 and G2 and whose arrow collection is the union of the arrow collections of G1 and G2 is also a grounding graph for p.
- Truncation: If G is a grounding graph for p, then any subgraph of G that is a candidate grounding graph and that has the property that if it contains any one of G's arrows to q then it contains all of G's arrows to q is a grounding graph.
The following is very controversial but very helpful:
- Well-foundedness: No grounding graph contains an infinite chain of arrows.
I think that if we reject well-foundedness, we should reject acyclicity. For the most plausible putative counterexamples will be infinitely nested propositions like p1&(p2&(p3&...)). But if we accept such propositions, we will also accept p&(p&(p&...)), and these will be cyclically grounded if the former will be non-well-foundedly grounded. But we shouldn't reject acyclicity, so we should accept well-foundedness, and if there are such infinitely nested propositions, we should ground them all at once in the symmetric conjunction p1&p2&... which then is grounded in each of its conjuncts.
Finally, we want to say something about how this interacts with logic. Say that p is free of q provided that is a fundamental grounding graph for p that does not contain q.
- Disjunction introduction: If p is free of (p or q), then the following is a grounding graph: p→(p or q).