  The theory of general relativity, as well as early developments in  atomic physics, clearly tell us, Weyl (1921b) suggests, that matter  uniquely determines the field, and that there exist deeper underlying physical laws with which modern physics, such as quantum theory, is  concerned, which specify “how the field is affected by  matter”. That is, experience tells us that matter plays the  role of a causal agent which uniquely determines the field,  and which therefore has an independent physical reality that cannot  be reduced to the field on which it acts. Weyl (1921b, 1924e) refers  to his theory of matter as the Agenstheorie der Materie  (literally, agent-theory of matter): 