To employ a comparison, I will say in regard to this concomitance, which I hold to be true, that it is like several bands of musicians or choirs separately taking up their parts and placed in such a way that they neither see nor hear one another, though they nevertheless, agree perfectly in following their notes, each one his own, in such a way that he who hears the whole finds in it a wonderful harmony much more surprising than if there were a connection between the performers. It is quite possible also that a person who is close by one of two such choirs could judge from the one what the other was doing, and would form such a habit (particularly if we supposed that he was able to hear his own choir without seeing it and to see the other without hearing it), that his imagination would come to his aid and he would no longer think of the choir where he was, but of the other, and he would take his own for an echo of the other, attributing to his own only certain interludes, in which certain rules of symphony by which he understood the other did not appear, or else attributing to his own certain movements which he caused to be made from his side, according to certain plans that he thought were imitated by the other because of the inter-relationship which he found in the kind of melody, not knowing at all that those who were in the other choir were doing also something which corresponded according to their own plans.

Nevertheless, I do not at all disapprove of the statement that minds are in some sort the occasional and even real causes of certain movements in the body, for, with regard to the divine resolves, whatever God has foreseen and pre-established with regard to minds, has been an occasion for his thus regulating the body from the very start, so that they might fit in together, each following the laws and forces that he has given them; and as the state of one is an unfailing consequence of the other, although frequently contingent and even free, we can say that God has established a real connection in virtue of this general conception of substances, which brings it about that they express one another perfectly. This connection, however, is not immediate, being founded only upon what God has given them in creating them.

If my opinion, that substances require a true unity, is founded only upon a definition which I have made up contrary to the common usage, this would be a mere question of words; but besides the fact that most philosophers have understood this term in nearly the same way, namely, that "a distinction should be made between unity through itself and unity through accident, between substantial form and accidental form, between an imperfect and a perfect compound, between natural and artificial," I take still higher ground and, leaving the question of terminology, I believe that where there are only beings by aggregation, there are not even real beings, because every being by aggregation pre-supposes beings endowed with true unity, because it obtains its reality only from the reality of the elements of which it is composed, so that it will have no reality at all if every being of which it is composed is again a being by aggregation; or else we must seek some other foundation for its reality, seeing that by this method it can never be reached, even by searching forever. I grant, M., that in all corporeal nature there exist only machines (some of which are alive), but I do not grant that there exist only aggregations of substances, and if there do exist aggregations of substances it must be that there are also real substances of which all these aggregations are the product; we therefore come necessarily to the mathematical points out of which certain writers have constructed extension, or to the atoms of Epicurus and of M. Cordemoy- things which you reject quite as much as I do; or else we must acknowledge that no reality is to be found in bodies. The other alternative is to say that there are certain substances which have a real unity. I have already said in another letter that the composite of the diamonds of the Grand Duke and of the Great Mogul could be called a pair of diamonds, but this would only be a being of the reason, and if they were brought together they would become a being of the imagination or perception, that is to say, a phenomenon, because contact, common movement and even agreement in design, do not effect a substantial unity. It is true that sometimes there is more and sometimes less basis for supposing that several things constitute one, according as the things have more or less connection, but this is only a means to abbreviate our thinking and to represent the phenomenon.

It seems also that what constitutes the essence of a being by aggregation consists solely in the mode of the being of its component elements. For example, what constitutes the essence of an army? It is simply the mode of being of the men who compose it. This mode of being presupposes, accordingly a substance of which the essence is not a mode of being of a substance. Every machine therefore presupposes some substance in the parts out of which it is made, and there is no plurality without true unities; in short, I consider as an axiom this identical proposition, which receives two meanings only through a change in accent; namely, that what is not truly a being is not truly a being. It has always been thought that one and being are reciprocal terms. Being is very different from beings, but the plural presupposes the singular; and there where there is no being, are there still less several beings. What can be clearer? I thought, therefore, that I should be permitted to distinguish beings by aggregation from substances, since these beings have their unity only in our minds, and our minds repose upon the relations or the modes of real substances. If a machine is a substance, a circle of men who are holding hands would be one also, so an army, and in fact, any gathering together of substances. I do not say that there is nothing substantial or nothing but appearance in things which have not a true unity, for I acknowledge that they have as much of reality or substantiality as there is of true unity in that which enters into their composition.