  We examine the necessary physical underpinnings for setting up the
cosmological standard model with a global cosmic time parameter. In particular,
we discuss the role of Weyl's principle which asserts that cosmic matter moves
according to certain regularity requirements. After a brief historical
introduction to Weyl's principle we argue that although the principle is often
not explicitly mentioned in modern standard texts on cosmology, it is
implicitly assumed and is, in fact, necessary for a physically well-defined
notion of cosmic time. We finally point out that Weyl's principle might be in
conflict with the wide-spread idea that the universe at some very early stage
can be described exclusively in terms of quantum theory.
