In this chapter I shall look at two potential obstacles to the libertarian's view that, in order for us to be free and responsible agents, our future must be physically open – a “garden of forking paths”, as it has been described. The first obstacle is an argument by Harry Frankfurt designed to refute what he terms the “principle of alternate possibilities”. The second obstacle is the special theory of relativity, which appears to imply that the future simply cannot be a garden of forking paths.

The principle of alternate possibilities

When I accept moral responsibility for an act of mine I am admitting that I knew the likely moral consequences of the act before I carried it out and, knowing this, that I freely chose to carry out that act. Implicit in this is the understanding that I could have refrained from carrying out the act. I could have done otherwise. And if I could not have refrained from carrying out the act, if I could not have done otherwise, then I cannot be held responsible for my act.