What guarantee have we for the existence of material objects affecting our senses? That the ideas of sense do not come from ourselves, is shown by the fact that it is not in our power to determine the objects which we perceive, or the character of our perception of them. The supposition that God has caused our perceptions directly, or by means of something which has no resemblance whatever to an external object extended in three dimensions and movable, is excluded by the fact that God is not a deceiver. In reliance on God's veracity we may accept as true whatever the reason declares concerning body, though not all the reports of the senses, which so often deceive us. At the instance of the senses we clearly and distinctly perceive matter distinct from our mind and from God, extended in three dimensions, length, breadth, and depth, with variously formed and variously moving parts, which occasion in us sensations of many kinds. The belief that perception makes known things as they really are is a prejudice of sense to be discarded; on the contrary, it merely informs us concerning the utility or harmfulness of objects, concerning their relation to man as a being composed of soul and body. (The body is that material thing which is very intimately joined with the mind, and occasions in the latter certain feelings, _e.g._, pain, which as merely cogitative it would not have.) Sense qualities, as color, sound, odor, cannot constitute the essence of matter, for their variation or loss changes nothing in it; I can abstract from them without the material thing disappearing.[1] There is one property, however, extensive magnitude (_quantitas_), whose removal would imply the destruction of matter itself. Thus I perceive by pure thought that the essence of matter consists in extension, in that which constitutes the object of geometry, in that magnitude which is divisible, figurable, and movable. This thesis (_corpus = extensio sive spatium_) is next defended by Descartes against several objections. In reply to the objection drawn from the condensation and rarefaction of bodies, he urges that the apparent increase or decrease in extension is, in fact, a mere change of figure; that the rarefaction of a body depends on the increase in size of the intervals between its parts, and the entrance into them of foreign bodies, just as a sponge swells up when its pores become filled with water and, therefore, enlarged. The demand that the pores, and the bodies which force their way into them, should always be perceptible to the senses, is groundless. He meets the second point, that we call extension by itself _space_, and not body, by maintaining that the distinction between extension and corporeal substance is a distinction in thought, and not in reality; that attribute and substance, mathematical and physical bodies, are not distinct in fact but only in our thought of them. We apply the term space to extension in general, as an abstraction, and body to a given