138. Passive sensibility, active sensibility, understanding, and will, are, if we be not mistaken, all that our understanding contains; and this is why we cannot conceive of any attribute characteristic of being, except these four. Let us examine these, each in its turn, and with the care required by the importance of the results which will follow this demarcation. 139. By passive sensibility we understand the form under which bodies are presented. As we have already explained it in several places, this form is reducible to figured or bounded extension. It cannot be denied that this attribute contains a true determination, as there is nothing more determinate than objects presented to our senses, with extension, and figure, and other properties annexed to these fundamental attributes. Motion and impenetrability are determinations which accompany extension, or rather they are relations of extension. To us, motion is the change of the situations of a body in space, or the alteration in the positions of the extension of a body, with respect to the extension of space. Impenetrability is the reciprocal exclusion of two extensions. The idea of solid and liquid, of hard and soft, and other similar ideas, express relations of the extension of a body to their admission, with greater or less resistance, of the extension of another in one and the same place. Questions upon the nature of extension have no place here. Extension is, so far as we are concerned, a determinate object, presented to us in the clearest intuition. The attribute of passive sensibility has ever been regarded as one of the most characteristic determinations; and this is why it has been made to enter as a fundamental classification in the scale of beings. The distinctions of corporeal and incorporeal, of material and immaterial, of sensible and insensible, are of as frequent use in ordinary language as in that of the schools; and it is obvious that the words, corporeal, material, and sensible, although not perfectly synonymous under some aspects, are usually taken to be such, in so far as they express a kind of beings, whose characteristic properties are those forms under which they are offered to our senses.