19 - The Nature of Possibility

Summary

ALETHIC MODALITY

We use alethic modal language all the time. For instance, we say that someone did not do something she could have done, or that the existence of unicorns is possible, or that 2 + 2 = 4 could not have failed to be true. We make counterfactual assertions such as “Were I to drop this glass, which in fact I do not, it would fall.” We think it might have been the case that Hitler had never existed. In these locutions we are speaking about situations and things that are not actual, of ways the universe might have been but was not.

Moreover, alethic modal language could not play the kind of role it does in our lives if we did not take a realist stance toward it. For instance, to decide rationally between alternatives, we often need to consider what consequences would result from each alternative. To decide questions of moral responsibility we often need to decide what else could have been done. The laws of nature by which we navigate the world have counterfactual force. If we did not take our alethic modal claims to express objective truths, modal language could not play the role it does in these cases.

A useful way of clarifying modal discourse is to introduce the notion of a possible world, or world for short, which is a complete way that a universe might have been.