  Metaphysics is the science of being and asks the question ``What really
exists?'' The answer to this question has been sought for by mankind since the
beginning of recorded time. In the past 2500 years there have been many answers
to this question and these answers dominate our view of how physics is done.
Examples of questions which were originally metaphysical are the shape of the
earth, the motion of the earth, the existence of atoms, the relativity of space
and time, the uncertainty principle, the renormalization of field theory and
the existence of quarks and strings. I will explore our changing conception of
what constitutes reality by examining the views of Aristotle, Ptolemy, St.
Thomas Aquinas, Copernicus, Galileo, Bacon, Descartes, Newton, Leibnitz,
Compte, Einstein, Bohr, Feynman, Schwinger, Yang, Gell-Mann, Wilson and Witten.
