Change

In the modern era a fundamental reorientation in the theory of motion occurred when the principle of inertia was recognized. Put forth first by René Descartes (1596–1650) and Pierre Gassendi (1592–1655), and canonized by Isaac Newton (1642–1727) as his first law, the principle stated that a body in uniform straight motion tends to stay in motion, and a body at rest tends to stay at rest. Thus the physicist did not need to explain continued motion. Moreover, since Newton's law of gravity could account for elliptical motions of satellites around a massive body, there was no longer any need for an unmoved mover to maintain the cosmic order. The kind of kinetic change that needed to be explained was acceleration, which depended on the application of a force to the moving body. John Locke (1632–1704) identified motion and rest as primary qualities whose ideas resembled the originals; colors and the like were secondary qualities whose ideas did not resemble the originals. Increasingly, motion in place came to be seen as the fundamental kind of change, which accounted for all other kinds.

History and physics.

Up until the nineteenth century, the study of history had been of little interest to philosophers. Aristotle had said that history was less philosophical than poetry, because it dealt with the particular rather than the universal. But when Georg Wilhelm Friedrich Hegel (1770–1831) explained the development of consciousness as a dialectical progression leading to ever more comprehensive concepts, he made history vitally important to philosophy. In particular the history of thought seemed to reflect the growth of spirit, exemplifying the realization of freedom and self-consciousness. An understanding of the development of culture and human institutions was now indispensable for philosophy. Historical change was essential to human self-realization. Becoming was the fulfillment of the concepts of being and not-being.

Although the historicist idealism launched by Hegel was highly influential, it eventually occasioned a strong reaction in England. One of the leading critics of idealism, Bertrand Russell (1872–1970), argued against "internal relations" by which every event was necessarily connected with every other. Instead, he proposed that the world consisted of a set of independent facts. Change itself could be defined in terms of propositions: "Change is the difference, in respect of truth or falsehood, between a proposition concerning an entity and a time T, and a proposition concerning the same entity and another time T (, provided that the two propositions differ only by the fact that T occurs in one where T (occurs in the other" (sec. 442). Thus if "Socrates is literate at T" is false and "Socrates is literate at T (" is true for the times 465 B.C.E. and 450 B.C.E., respectively, we can infer a change. Against Russell, John M. E. M'Taggart (1866–1925) has argued that we should distinguish between an "A series" of events ordered with respect to past, present, and future, and a "B series" ordered with respect to before and after; only the former entails the existence (the flow) of time, since the latter involves only a fixed sequence of determinate events. But the A series produces contradiction, since every event is allegedly past, present, and future, that is, has incompatible predicates. Thus the A series is incoherent, and consequently there is no time and hence no change. Yet one can reply that the B series provides an adequate basis for time and change, and that the predicates past, present, and future are no more incompatible than taller and shorter: they are incomplete predicates, which, when properly completed for a given subject, would not produce a contradiction: "past with respect to time T ₃ [or event E ₃, etc.]," "present with respect to T ₂," "future with respect to T ₁." The kinds of changes described by Russell's and M'Taggart's accounts (sometimes called "Cambridge changes") have been criticized as too weak: a statement about a subject can change truth value with the subject's undergoing any alteration. For instance, I become shorter than my son when he grows taller than I, even though my stature does not change. Russell could reply that his account is meant only to identify some change in the world, not to analyze the subject of that change. In any case, we seem to need a richer account than Russell's to analyze the structure of change itself.

Advances in physics have affected views of the place of change in the world. Whereas Newtonian physics saw motion and rest as interchangeable phenomena seen from different frames of reference, the Theory of Relativity puts a limit on speed (the speed of light) and makes time a fourth dimension on a par with the three dimensions of space. Space, time, and mass all became relative quantities, and acceleration ceased to provide a necessary condition of change. At a quantum level, some particles have properties of both bodies and waves, and subatomic particles seem to be packets of energy. New theories such as String Theory suggest that subatomic particles are states of multidimensional strings of energy. Alfred North Whitehead (1861–1947) drew on early-twentieth-century physics to develop a speculative metaphysics positing processes rather than things as the ultimate realities. His style of philosophy, however, has gone out of favor, and in the late twentieth century philosophers tended to study change in trying to specify identity conditions for events.