Now this cause cannot be contained in the actual nature of man, for the true definition of man does not involve any consideration of the number twenty.

Consequently, the cause for the existence of these twenty men, and, consequently, of each of them, must necessarily be sought externally to each individual.

Hence we may lay down the absolute rule, that everything which may consist of several individuals must have an external cause.

And, as it has been shown already that existence appertains to the nature of substance, existence must necessarily be included in its definition; and from its definition alone existence must be deducible.

But from its definition (as we have shown, notes ii., iii.), we cannot infer the existence of several substances; therefore it follows that there is only one substance of the same nature.