We must also show that while the rational soul makes use of the data of sense perception, which are corporeal, as the occasions for the formation of its general ideas, it is not wholly dependent upon them, and the sense data alone are inadequate to give the soul its intellectual truths. Empirical knowledge is inductive, and no induction can be more general and more certain than the particular facts from which it is derived. As all experience, however rich, is necessarily finite, empirical knowledge is never universally certain. But the soul does possess universally certain knowledge, as for example the truths of mathematics and logic; hence the origin of these truths can not be empirical. How does the soul come to have such knowledge? We must assume that there is a divine emanation cleaving to the soul, which stands to it in the relation of light to the sense of sight. It is to the illumination of this intellectual substance and not to the data of sense perception that the soul owes the universal certainty of its knowledge. This divine substance is the _Active Intellect_. As long as the soul is united with the body, perfect union with the Active Intellect is impossible. But as the soul becomes more and more perfect through the acquisition of knowledge, it cleaves more and more to the Active Intellect, and this union becomes complete after death. Thus the immortality of the soul is proved by reason. It is based upon the conviction that the soul is an immaterial substance and that its perfection lies in its acquisition of intellectual ideas.[208] Judah Halevi cannot help admitting the fascination such speculation exercises upon the mind of the student. But he must warn him against being misled by the fame of such names as Plato and Aristotle, and supposing that because in logic and mathematics the philosophers give us real proofs, they are equally trustworthy in metaphysical speculation. If the soul is, as they say, an intellectual substance not limited in place and for this reason not subject to genesis and decay, there is no way to distinguish one soul from another, since it is matter which constitutes individual existence. How then can my soul be distinguished from yours, or from the Active Intellect and the other Intelligences, or from the First Cause itself? The souls of Plato and Aristotle should become one so that the one should know the secret thoughts of the other. If the soul gets its ideas through divine illumination from the Active Intellect, how is it that philosophers do not intuit their ideas at once like God and the Active Intellect, and how is it they forget?