a. Proof through Authorities of Philosophers

Aristotle, On the Soul 2.2.413b25-27, says that “the intellect is separated from other things as the perpetual from the corruptible.” If it be said that it is separated as to its operation, on the contrary: from this the proposed conclusion follows, that if it can be separated as to operation then as to being as well (according to Aristotle On the Soul 1.1.403a7-12).

94. Again, On the Soul 3.4.429a29-b5, a difference is set down between sense and intellect, that “a surpassing sensible object destroys the sense”, and so, after the sensation, the sense perceives a lesser sensible thing less. But it is not so with the intellect; rather after it has understood things supremely intelligible, it understands lesser things more; therefore the intellect is not weakened in its operation; and then it follows further that it is incorruptible in its being.

95. Again Metaphysics 12.3.1070a21-27, “Moving causes, just as they exist beforehand, are yet as the rational nature (that is as the form) simultaneous with the caused thing as a whole. For when a man is being healed, health exists then also. But whether anything remains afterwards needs to be examined. For nothing prevents this in some cases, as suppose the soul is such - not every soul, but the intellect     etc .” The Philosopher means to say, then, that the intellect is a form that remains after the composite but not beforehand.

96. Again Generation of Animals 2.3.736b27-28, “It remains then that only the intellect comes from without.” Therefore     , it does not receive its being through generation but from an extrinsic cause. And, consequently, it cannot receive non-being through corruption or through any other inferior corruptive cause, because its being is not subject to any such cause, for it is immediately from a superior cause.

97. Again, a number of reasons can be formed from the Authorities of the Philosopher [3.18, n.45].

There is one principle the Philosopher has that ‘natural desire cannot be vain’; but there is a natural desire now in the soul to exist always.

98. Again, in Metaphysics 7.15.1039b29-30 he maintains that ‘matter is that whereby a thing can be and not be’; therefore, according to him, what does not have matter does not have the possibility not to exist; the intellective soul, according to him, does not have matter, because it is a simple form.

99. Again, in Ethics 3.9.1115a32-b1 he maintains that a brave man should expose himself to death for the sake of the republic, and he maintains the same in Ethics 9.8.1169a18-20, and speaks according to the judgment of natural reason. Therefore, the immortality of the soul can be known according to natural reason. The proof of this consequence is that no one should or can desire his own complete non-existence for any good of virtue, whether a good in himself or in another or of the republic. For, according to Augustine On Free Choice of the Will 3.7-8 nn.68-84, non-existence cannot be desired; but now, if the soul were not immortal, someone would get, by dying, total nonexistence.