Abstract
In the introductory treatise to his main philosophical work, "The Book of Beliefs and Opinions", Saadia Gaon analyses the main causes of doubts arising in the minds of humans in their quest for truth, and defends the use of reason even concerning questions of religion. However, in order to achieve certainty regarding truth it is necessary to employ all the resources of human knowledge: the three natural sources, i.e. sensation, intuition, and logical inference, as well as the fourth, which is the authentic tradition, both Biblical and rabbinic, as granted to the Jewish people.