  This paper argues that the physics of condensed matter cannot be fully
reduced to the supposedly fundamental quantum mechanical theory for all the
atoms of which the system consists. In fact, there are many reasons to reject
the idea that the world of physics is causally closed with everything being
determined bottom-up by fundamental microscopic laws. This is illustrated by
considering how condensed-matter theory is done in practice. It is never done
by starting with a microscopic theory for the interaction of all the atoms of
the system. Instead, approximations, plausible assumptions, intuitive models,
and phenomenological theories are used to mathematically describe and explain
the properties of systems that consist of a macroscopic number of particles. I
argue that this is not merely a matter of convenience, but that there are
fundamental and qualitative differences or even contradictions between the
microscopic theory and the theory that is used in practice. The paper includes
a list of arguments in favor of strong emergence and top-down causation within
the realm of physics, and a response to several widespread objections against
this view.
