Promising against the Evidence

We often promise to phi despite having evidence that there is a significant chance that we won't phi. This gives rise to a pressing philosophical problem: Are we irresponsible in making such promises since, it seems, we are insincere or irrational in making them? I argue that we needn't be. When it's up to us to phi, our practical reasons for phi-ing partly determine whether it is rational for us to believe that we will phi. That is why we can sometimes rationally believe that we will phi even if our belief goes against the evidence.