"If it were possible to see what makes extension, that kind of extension which falls under our eyes at present would vanish, and our minds would perceive nothing else than simple realities existing in mutual externality to one another. It would be as if we could distinguish the minute particles of matter variously disposed from which a painted image is formed: if we could do it, the image, which is nothing but a phenomenon, would vanish. . . . If we think of two simple realities as both existing at the same time, but distinct from one another, we look at them as if they were outside of one another, and hence conceive them as extended." The monads are outside of one another, not spatially, but ideally; but this reciprocal distinction from one another, if it is to appear in phenomenal mode, must take the form of an image, and the image is spatial. But if the monads were pure activity, they would _not_ take phenomenal form or appear in an image. They would always be thought just as they are,--unextended activities realizing the spiritual essence of the universe. But they are not pure activity; they are passive as well. It is in virtue of this passive element that the ideal externality takes upon itself phenomenal or sensible form, and thus appears as spatial externality. Leibniz, in a passage already quoted, refers to the diffusion of materiality or _antitypia_. This word, which is of frequent occurrence in the discussions of Leibniz, he translates generally as "impenetrability," sometimes as "passive resistance." It corresponds to the solidity or resistance of which Locke spoke as forming the essence of matter. Antitypia is the representation by a monad of the passive element in other monads. Leibniz sometimes speaks as if all created monads had in themselves antitypia, and hence extension; but he more accurately expresses it by saying that they need (_exigent_) it. This is a technical term which he elsewhere uses to express the relation of the possible to the actual. The possible "needs" the actual, not in the sense that it _necessarily_ requires existence, but in the sense that when the actual gives it existence, it is the logical basis of the actual,--the actual, on the other hand, being its real complement. The passivity of the monad is therefore at once the logical basis and the possibility of the impenetrability of matter. It is owing to the passivity of the monad that it does not adequately reflect (that it is not transparent to, so to speak) the activities of other monads. In its irresponsiveness, it fails to mirror them in itself. It may be said, therefore, to be impenetrable to them. They in turn, so far as they are passive, are impenetrable to it. Now the impenetrable is, _ex vi terminis_, that which excludes, and that which excludes, not in virtue of its active elasticity, but in virtue of its mere inertia, its dead weight, as it were, of resistance. But mutual exclusion of this passive sort constitutes that which is extended.