A philosophy usually begins with a cosmology—begins when there is a presumption of some kind of form and with a concept of substance. It begins with a reality, a being. Whatever else Heraclitus may have given philosophy, he is most important for giving to all philosophies the demanding questions, “What in any reality is moved?” and “Where in any reality is there change?” He is answered by an opponent who denies change and asserts only what is and that it is because it is permanent and fixed. So Heraclitus is really responsible for Parmenides. Once this confrontation has been effected, no philosopher and no scientist can ignore the application of the problem of change to the realities of his subject. The scientist sees change and motion in all the composite and simple particulars of nature: changes of state produce spectra, and activity produces myriads of records in cloud chambers, bubble chambers, and photographic emulsions. There are also other motions: electron spins, nuclear spins, particle decay, radioactivity, fission, and fusion. But modern physical science also depends on what is fixed in such concepts as bonding, the geometry of molecules, and other fixed aspects of the structure of matter. While it is true that activity and change produce the most startling and insisting demands upon our interest, the unchanging character of molecules, of atoms, and of certain particles provides the fixed framework in which activity itself can be studied.