109. The idea of substance and all its applications, as well to the external as to the internal world, are far from leading us to infer the existence of a _single_ substance; on the contrary, reason according with experience forces us to acknowledge a _multitude_ of substances. Why should we admit only one substance? This is one of the most important questions of philosophy, and from the most ancient times has given occasion to the most serious errors; it consequently deserves a careful investigation. 110. Those who admit only one substance must found their opinion either on the idea of substance or on experience; our mind can have no other recourse than to its primitive ideas, or the teachings of experience. Let us begin with the _a priori_ method or that which is founded on the idea. 111. What do you understand by substance? we ask. If by substance you understand a being subsisting by itself, and by this subsistence you mean that it has no need of another, and never had any need of another in order to exist, then you are speaking of a being that is _not caused_, of a necessary being which has in itself the sufficient and necessary reason of its existence. If you say this being is only one, or that there is no other of its kind, we agree with you, only we tell you that you take the name of substance in an improper sense. But at bottom the difference would be only in the name; and in order to come to a mutual understanding it is only necessary for us to know that by substance you understand an absolutely necessary, and consequently absolutely independent being. But if you assert that this being is the only one in the sense that there is nothing, and can be nothing beside it, then your assertion is gratuitous and we ask for joint proof. Why should the necessary being exclude the possibility of other beings? Is it not more reasonable to conclude that it contains the reason of their possibility and existence? The being which has in itself the necessity of existing, must possess activity, and the external term of this activity is production. Why may not other beings be the result of this production? Inasmuch as produced they would be distinct from the being producing them.