I [am] obliged to recur ultimately to my habitual anodyne, I feel: therefore I exist. I feel bodies which are not myself: there are other existencies then. I call them matter. I feel them changing place. This gives me motion. Where there is an absence of matter, I call it void, or nothing, or immaterial space. On the basis of sensation, of matter and motion, we may erect the fabric of all the certainties we can have or need.