Let us, then, consider on what foundation this supposition is raised: let us examine if it contains any thing contradictory, and afterwards we shall discover what consequences may be derived from it. Though our senses are only judges of the external parts of bodies, we perfectly comprehend external affection and different figures. We can also imitate Nature, by representing external figures by different modes, as by painting, sculpture, and moulds; but although our senses are only judges of external qualities, we know there are internal qualities, some of which are general, as gravity. This quality, or power, does not act relatively to surfaces, but proportionably to the masses, or quantities of matter; there is, therefore, very active qualities in Nature, which even penetrate bodies to the most internal parts; but we shall never gain a perfect idea of these qualities, because, not being external, they cannot fall within the compass of our senses; but we can compare their effects, and deduce analogies therefrom, to answer for the effect of similar qualities. If our eyes, instead of representing to us the surface of objects only, were so formed as to shew us the internal parts alone, we should then have clear ideas of the latter, without the smallest knowledge of the former. In this supposition the internal moulds, which I have supposed to be made use of by Nature, might be as easily seen and conceived as the moulds for external figures. In that case we should have modes of imitating the internal parts of bodies as we now have for the external. These internal moulds, although we cannot acquire, Nature may be possessed of, as she is of the qualities of gravity, which penetrate to the internal particles of matter. The supposition of these moulds being formed on good analogies it only remains for us to examine if it includes any contradiction. It may be argued that the expression of _an internal mould_ includes two contradictory ideas; that the idea of a mould can only be related to the surface, and that the internal, according to this, must have a connection with the whole mass, and, therefore, it might as well be called a massive surface as an internal mould. I admit, that when we are about to represent ideas which have not hitherto been expressed, we are obliged to make use of terms which seem contradictory; for this reason philosophers have often employed foreign terms on such occasions, instead of applying those in common use, and which have a received signification; but this artifice is useless, since we can shew the opposition is only in the words, and that there is nothing contradictory in the idea. Now I affirm that a simple idea cannot contain a contradiction, that is, when we can form an idea of a thing; if this idea is simple it cannot be compounded; it cannot include any other idea, and, consequently, it will contain nothing opposite nor contrary.