Like Israeli Saadia shows considerable familiarity with Aristotelian notions as found in the Logic, the Physics and the Psychology. It is doubtful, however, whether he really knew Aristotle's more important treatises at first hand and in detail. The "Categories," a small treatise forming the first book of Aristotle's logic, he no doubt knew, but the other Aristotelian concepts he probably derived from secondary sources. For while he passes in review all the ten categories showing that none of them is applicable to God,[42] we scarcely find any mention of such important and fundamental Aristotelian conceptions as matter and form, potentiality and actuality, the four causes, formal, material, efficient and final—concepts which as soon as Aristotle began to be studied by Al Farabi and Avicenna became familiar to all who wrote anything at all bearing on philosophy, theology, or Biblical exegesis. Nay, the very concepts which he does employ seem to indicate in the way he uses them that he was not familiar with the context in which they are found in the Aristotelian treatises, or with the relation they bear to other views of Aristotle. Thus no one who knew Aristotle at first hand could make the mistake of regarding his definition of the soul as making the latter an accident.[43] When Saadia speaks of six kinds of motion [44] instead of three, he shows clearly that his knowledge of the Aristotelian theory of motion was limited to the little of it that is contained in the "Categories."
We are thus justified in saying, that Saadia's sources are Jewish literature and tradition, the works of the Mutakallimun, particularly the Muʿtazilites, and Aristotle, whose book on the "Categories" he knew at first hand.
Saadia tells us he was induced to write his book because he found that the beliefs and opinions of men were in an unsatisfactory state. While there are some persons who are fortunate enough to possess the truth and to know that they have it and rejoice thereat, this is not true of all. For there are others who when they have the truth know it not, and hence let it slip; others are still less fortunate and adopt false and erroneous opinions, which they regard as true; while still others vacillate continually, going from one opinion and belief to another. This gave him pain and he thought it his duty to make use of his limited knowledge to help them. A conscientious study of his book will tend to remove doubt and will substitute belief through knowledge for belief through tradition. Another result of such study, not less important, will be improvement of character and disposition, which will affect for the better a man's life in every respect, in relation to God as well as to his fellowmen.[45]
One may ask why it is that one encounters so many doubts and difficulties before arriving at true knowledge. The answer is, a human being is a creature, i. e., a being dependent upon another for its existence, and it is in the nature of a creature as such that it must labor for the truth with the sweat of its brow. For whatever a man does or has to do with is subject to time; each work must be accomplished gradually, step by step, part by part, in successive portions of time. And as the task before him is at the beginning complex, he has to analyze and simplify it. This takes time; while certainty and knowledge cannot come until the task is accomplished. Before that point is reached he is naturally in doubt.[46]
The sources of truth are three. First is that to which the senses testify. If our normal sense perceives under normal conditions which are free from illusion, we are certain of that perception.
The judgment is another source of truth. There are certain truths of which we are certain. This applies especially to such judgments of value, as that truth is good and falsehood is bad. In addition to these two sources of immediate knowledge, there is a third source based upon these two. This is logical inference. We are led to believe what we have not directly perceived or a matter concerning which we have no immediate knowledge of the second kind, because we infer it from something else which we have perceived or of which we have immediate certainty. Thus we believe man has a soul though we have never seen it because we infer its presence from its activity, which we do see.
These three sources are universal. They are not peculiar to a given race or religious denomination, though there are some persons who deny the validity of some or all of them. We Jews believe in them and in still another source of truth, namely, authentic tradition.[47]
Some think that a Jew is forbidden to speculate or philosophize about the truths of religion. This is not so. Genuine and sincere reflection and speculation is not prohibited. What is forbidden is to leave the sacred writings aside and rely on any opinions that occur to one concerning the beginnings of time and space. For one may find the truth or one may miss it. In any case until a person finds it, he is without a religious guide; and if he does find what seems to him the truth and bases his belief and conduct upon it, he is never sure that he may not later be assailed by doubts, which will lead him to drop his adopted belief. But if we hold fast to the commandments of the Bible, our own ratiocination on the truths of religion will be of great benefit to us.[48]
Our investigation of the facts of our religion will give us a reasoned and scientific knowledge of those things which the Prophets taught us dogmatically, and will enable us to answer the arguments and criticisms of our opponents directed against our faith. Hence it is not merely our privilege but our duty to confirm the truths of religion by reason.[49]
Here a question presents itself. If the reason can discover by itself the truths communicated to us by divine revelation, why was it necessary to have recourse to the latter? Why was it not left to the reason alone to guide us in our belief and in our conduct? The answer is, as was suggested before, that human reason proceeds gradually and does not reach its aim until the end of the process. In the meantime one is left without a guide. Besides not everybody's reason is adequate to discover truth. Some are altogether incapable of this difficult task, and many more are exposed to harassing doubts and perplexities which hinder their progress. Hence the necessity of revelation, because in the witness of the senses all are equally at home, men and women, young and old.[50]