Chapter 4

The proofs of the simplicity of the soul from its operations in general

435. Finally, we can prove the simplicity of the soul because this simplicity is the single, sufficient reason for explaining its different operations. This can be done in three ways. We can demonstrate that the efficient principle of such operations must be simple from 1. their nature, through the manifest opposition between what is extended and the principle which has the extended as its term; 2. their mode, through the opposition between the extrasubjective phenomena included in the concept of matter and the subjective phenomena which pertain solely to the sentient subject; 3. their term, through the opposition between the multiplicity of subjective phenomena and the unicity of their principle.
There are indeed many proofs of the simplicity of the soul.
Each operation of the soul, when carefully examined, provides three proofs because we can show that the soul is simple by considering the nature, mode and term of these operations.

436. Indeed, once it has been demonstrated that a given operation cannot be produced except by a simple principle, this principle can no longer contain in itself anything opposed to simplicity. If it could, it would no longer be the principle of that operation, as we suppose it to be. It cannot be simple and not simple at the same time. Let us grant that the principle contains in itself something not simple. This non-simple element is no longer the principle of that operation, but something else. It is not, therefore, the soul. A single operation necessarily requiring a simple principle is sufficient, therefore, to show that the soul is altogether simple.

437. Proof of the simplicity of the soul drawn from intellective operations is much easier for the unbiased mind to understand because those operations reveal themselves as obviously immune and pure from all material accretions. Even the ancient physicists, who clothed the soul with different bodily layers, as it were, of extremely fine atmospheric ether, were unanimous in recognising the mind as altogether incorporeal. For this precise reason, we begin from what is easiest, and first explain the proofs of the simplicity of the soul drawn from sensitive operations.(187) These proofs are of themselves quite sufficient to prove the simplicity of the human soul.

438. If it is shown that sensitive operations cannot be explained in any way without supposing them as effects of a simple cause, it is shown that the entire soul to which those operations pertain is simple. If the first sensitive principle, which is substantially identical in man with the first intellective principle, is simple, the human soul also must be simple because it is at one and the same time the first principle of feeling and knowing.

439. Lucretius himself felt the efficacy of this way of arguing, and tried to use it in favour of the mortality of the intellective soul which he deduces from the mortality of the sensitive soul:

To this I would reply that the sensitive soul does not die, but multiplies itself, as we shall see. In this case, neither does the intellective soul die. But our argument will be considerably strengthened if we put it like this. The sensitive soul is simple; therefore, the intellective soul also is simple. In other words, if the sensitive soul were extended and corporeal, it could be thought that perhaps the intellective soul might receive some extension and corporeity from it. But because the sensitive soul is unextended and incorporeal, it can be united to the intellective as simple to simple, without anything extended or corporeal arising from their union and identification.

Notes

(187) Italian philosophers such as Riccati and Garducci, have already used this fount to provide various proofs of the simplicity of the soul. Garducci shows, in the last of his Lettere ad un filosofo italiano, that bodily renewal is proof of the spirituality of the soul.