Contextual emergence is the idea that the environment, or context, of a system affects its behaviour by imposing constraints on the underlying physics. This means that knowing the microphysics is not enough to explain the system; you need to know the context too. But contextual emergence fails as a useful definition. The split between system and context is entirely arbitrary. Further, the context is a physical system too and all contextual emergence ends up saying is that different physical systems interact. This is illustrated with examples including the evolution of the universe, feedback and selection in chemical systems and chemical computation.