Full Idea
Physics is committed to arithmetic, which seems committed to abstract objects such as numbers, and its causal explanations seem to appeal to properties, such as mass and charge.
Clarification
Hence even physics must apparently accept the existence of things which are not obviously physical
Gist of Idea
Physics requires the existence of properties, and also the abstract objects of arithmetic
Source
Georges Rey (Contemporary Philosophy of Mind [1997], 2.3)
Book Reference
Rey,Georges: 'Contemporary Philosophy of Mind' [Blackwell 1997], p.46