According to Saadiah, belief or faith must be an integral part of our soul; the various truths which form the faith are stored up in the soul as in a repository, completely ready for use whenever required. It is, however, possible that we store up opinions as true which are false. Tests must be applied to each opinion in order to ascertain its right character. Three of the tests are of a general nature, but the fourth has its force only for us, the believers in the truth of the Holy Writings. The first three tests will show us whether a certain opinion is confirmed or contradicted by our senses, by our innate ideas, or by our logical reasoning. In addition to these we possess a fourth test in the trustworthy communication (‏הגדה הנאמנת‎), i.e., the contents of Holy Writ and Tradition. Holy Writ recognises the necessity of the three general tests, and frequently exhorts us to apply them. On the other hand, Saadiah is convinced that the contents of Holy Writ and Tradition are never contradicted, but in many cases are confirmed by these tests. Such confirmation is in reality superfluous; but the human mind feels more at ease when it finds that the teaching of Holy Writ is supported by other proofs. Besides, attacks on the Bible come frequently from these tests, and it is therefore useful to learn how to refute them. According to Saadiah, the truth taught in the Bible can never be contradicted by the results of scientific or philosophical research. Thus to Saadiah philosophy and science are mere luxuries, and cannot be considered as handmaids to the Torah. They are not studied on account of their intrinsic value or as helps for the understanding of Holy Writ, but merely for the purpose of procuring proper weapons for theological warfare, or of superadding the conviction that what is known to us from the most trustworthy source is confirmed from other less reliable sources.