51 All things, indeed, are believed to be, either because of experience, or on grounds of reason, or from similar instances, or because it is fitting that they be, and each of these supports our belief. Experience teaches us that we are moved; reason, that which moves us must be considered the property of another power; similar instances show that the field has borne crops, and therefore we expect that it will continue to bear them. Fitness, because even where we do not think that there will be results, yet we believe that it is by no means fitting to give up the works of virtue.
Source: On the Death of Satyrus (New Advent)