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]