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In M. C. Galavotti, P. S uppes & D. Costantini , (Eds.), Stochastic Causality. Stanford, CA: CSLI Publications. 203-220. We a k and Strong Reversibility of Causal Processes* PATRICK SUPPES Stanford University It is widely known, and often commented upon, that the basic equations of classical physics remain valid under a transformation from tim e t to -t . It is often also rem arked that this invariance under a change of direction of tim e is completely contrary to ordinary experience. So the invariance of well- established classical physics, as well as other parts of physics, for example, relativistic mechanics and the Schrtidinger equation in quan tum mechanics, creates a natural philosophical tension about the nature of causal processes. Th e purpose of the present article is not to resolve in any complete way this tension between physics and ordinary experience, but to at least reduce the tension by introducing two con cepts of reversibility. The first is that of weak reversibility, which is what is exemplified in the in variance of the equations of classical physics under time reversal. This is the weak sense of reversibil- ity, because we can usually distinguish by observation whether a system of particle mechanics is running one way or the other. The appearance is not the same when we go to a rever sal of time. Elementary examples would be mea- suring the velocity of a particle accelerating from rest, moving in a straight 'I am indebted to Ubaldo Garibaldi for several useful comments on an earlier draft. Stochastic Causality. Maria Carla Galavotti, Panick Suppcs a nd Dom eniw Costan tini (eds.). Copyright @ 2001, CSLI Pub lications . 2 0 3

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