1. Impenetrability, which is pretended to be the definition of matter, is only an hypothesis of careless naturalists, a gross conclusion deduced from a superficial judgment. Experience shows that matter possesses infinite divisibility, infinite expansibility, porosity without assignable limits, and permeability by heat, electricity, and magnetism, together with a power of retaining them indefinitely; affinities, reciprocal influences, and transformations without number: qualities, all of them, hardly compatible with the assumption of an impenetrable aliquid. Elasticity, which, better than any other property of matter, could lead, through the idea of spring or resistance, to that of impenetrability, is subject to the control of a thousand circumstances, and depends entirely on molecular attraction: now, what is more irreconcilable with impenetrability than this attraction? Finally, there is a science which might be defined with exactness as the SCIENCE OF PENETRABILITY OF MATTER: I mean chemistry. In fact, how does what is called chemical composition differ from penetration?[5]. . . . In short, we know matter only through its forms; of its substance we know nothing. How, then, is it possible to affirm the reality of an invisible, impalpable, incoercible being, ever changing, ever vanishing, impenetrable to thought alone, to which it exhibits only its disguises? Materialist! I permit you to testify to the reality of your sensations; as to what occasions them, all that you can say involves this reciprocity: something (which you call matter) is the occasion of sensations which are felt by another something (which I call spirit). [5] Chemists distinguish between MIXTURE and COMPOSITION, just as logicians distinguish between the association of ideas and their synthesis. It is true, nevertheless, that, according to the chemists, composition may be after all but a mixture, or rather an aggregation of atoms, no longer fortuitous, but systematic, the atoms forming different compounds by varying their arrangement. But still this is only an hypothesis, wholly gratuitous; an hypothesis which explains nothing, and has not even the merit of being logical. Why does a purely NUMERICAL or GEOMETRICAL difference in the composition and form of atoms give rise to PHYSIOLOGICAL properties so different? If atoms are indivisible and impenetrable, why does not their association, confined to mechanical effects, leave them unchanged in essence? Where is the relation between the cause supposed and the effect obtained?