As Archimedes demanded only one fixed point in order to move the world, so Descartes desired to find one certain and indubitable principle upon which he could plant his feet and lift himself out of the universal doubt which environed him. He found it in the proposition--I exist. This for me is the most direct, immediate, and certain of all intuitions. I can not doubt, I can not deny my own existence. Whatever else I doubt, I can not doubt that I, the doubter, exist. This I that thinks, that is conscious, is the fundamental _reality_.[7] I see around me a plurality of personal existences who are self-conscious and self-manifesting beings--beings who think and feel, and display their activities in time and space, as I do; and I can no more doubt their existence than I can doubt my own. This combination of the content of external perception with that of internal perception gives the immediate consciousness of external _reality_.[8] Besides these personal existences analogous to my own, there are other objects which exist in relation to my corporeal organism--relations of position, distance, and direction, which are purely objective. These existences offer resistance to my muscular effort to displace them in space, and defy all my mental effort to reduce them to the category of subjective phenomena. These objects have specific properties or exist in certain conditions which, in their mutual relation with my sensitive organism, produce in me certain vital affections, as heat, light, color, and sound. These affections presuppose a force or energy outside of my consciousness, and distinct from myself. Thus I am constrained to believe that the earth on which I tread, the heavens that shine upon me, the forms and movements which surround me, are not vain shadows, unreal phantoms of my own creation, but real entities. The totality of existence called the universe is for me a _reality_. The phenomena of the universe are in ceaseless flow and change. Bodies are aggregated and dissolved. Plants are evolved from germs, they live and grow, then decay and perish. Animals and men are born and developed to maturity, then they sicken and die. The earth itself is in constant change. The storms of heaven, the erosion of the atmosphere, the gnawing of the tidal wave, the mountain torrent, the flowing river, the earthquake and the volcano, are perpetually changing the aspect of the globe. There is perpetual _genesis_, ceaseless _becoming_, incessant _change_.