All of this means that every body whose parts are only mechanically united is not a substance but only a machine or an aggregate of several substances.

I will begin with this last. And I will say frankly that it is only a dispute regarding a word. For St. Augustine did not hesitate to recognize that bodies have no real unity; because a unit should be indivisible and no body is indivisible. There is, therefore, no true unity excepting in Spirit, any more than there is a true Me outside of them. Now, what is your conclusion from that? "That there is nothing substantial in those bodies which have no soul or substantial form." In order that this conclusion may be valid we must first of all define substance and substantial in these terms, "I call substance and substantial that which has a true unity." But since this definition has not yet been received there is no philosopher who has not as much right to say, "I call substance that which is not modality or manner of being," and he could therefore maintain that it is untrue to say that there is nothing substantial in a block of marble, "because this block of marble is by no means a manner of being of another substance, and all that can be said of it is that it is not a single substance but several substances joined together mechanically." This philosopher would say "this is what seems to me paradoxical: that there should be nothing substantial in that which seems to be made up of several substances." He could add that he understood still less what you meant by the words "bodies would be without doubt something imaginary and only of appearance if they were composed only of matter and its modifications." For you postulate only matter and its modifications in everything that has no soul or no substantial, indestructible, indivisible and ingenerable form and it is only in the case of animals that you admit this class of forms. You will therefore be obliged to say that all the rest of nature is something imaginary and merely an appearance, and for a still stronger reason you would have to say the same thing of all the works of men.

I cannot agree to these latter propositions, but I see no objection to thinking that in every corporeal nature there is only a machine and an aggregate of substances, because of no one of its parts could one say strictly that it is a single substance. This serves merely to make evident what is worth while noticing, as St. Augustine has done, that the substance which thinks, or a spiritual substance, is through this fact much more excellent than extended or corporeal substance. The spiritual substance alone has a true unity and a true ego, while the corporeal substance does not have them. It follows from this, that this fact, that the body has no true unity when its essence is extension, cannot be put forward to prove that extension is not of the essence of the body; for, perhaps, the essence of the body has no true unity, as you grant in the case of all those which are not united to a soul or to a substantial form.

I do not know, M., what inclined you to believe that brutes have these souls or substantial forms, which, according to you, must be indivisible, indestructible and ingenerable. It is not because you consider it necessary to explain their actions, for you say expressly "that all the phenomena can be explained mechanically or by the corpuscular philosophy in accordance with certain postulated mechanical principles, without going into the question whether there are souls or not." It is also not because the bodies of brutes need to have a true unity and because they are not mere machines or aggregations of substances; if plants are merely the latter what necessity is there that brutes should be anything else? Further, it is not clear how this opinion can be easily maintained, if we consider these souls as indivisible and indestructible. What would be said of a worm, of which, when cut in two, both parts move off as before? If a house where a hundred thousand silk-worms were being kept should catch fire and burn up, what would become of those one hundred thousand indestructible souls? Would they exist apart from all matter like our souls? In the same way, what became of the souls of those millions of frogs which Moses caused to die when he stopped the plague? And of that innumerable number of quails which the Israelites killed in the desert or of all the animals which perished in the flood? There are also other embarrassing questions in regard to the condition of these souls in each brute at the moment that they are conceived. Is it that they are in seminibus? Are they there indivisible and indestructible? Quid ergo fit, cum irrita cadunt sine ullis conceptibus semina? Quid cum bruta mascula ad foeminas non accedunt toto vitae suae tempore? It will suffice to have indicated these difficulties.

There still remains the discussion of the unity which a reasoning soul has. It is agreed that it has a true and a perfect unity, a true Me, and that it communicates in some sort this unity and this Me to that composite whole of the soul and body which is called the man; for, although this whole is not indestructible because it perishes when the soul is separated from the body, it is indivisible in this sense, that half a man cannot be conceived of. In considering the body apart, however, in the same way that our soul does not communicate to it its indestructibility, we cannot see, properly speaking, that it communicates either its true unity or its indivisibility. Even though it be united to our soul, nevertheless, its parts are truly united among themselves only mechanically, and thus there is not a single bodily substance, but an aggregation of many corporeal substances. Not less true is it that it is quite as divisible as all the other bodies in nature. The divisibility, however, is inconsistent with unity, therefore it has no true unity. But you say, it acquires the unity through the soul, that is to say, because it belongs to a soul which is a true unit; this, however, is not an intrinsic unity in the body, but is like that of different provinces which are governed by a single king and thus constitute one kingdom.