Causal influences in the real world occurring on evolutionary, developmental, and functional timescales are characterised by a combination of bottom up and top down effects. Digital computers give very clear examplars of how this happens, and it is crucial to brain functioning. There are five different distinct classes of top down effects, the key one leading to the existence of complex systems being adaptive selection. The case will be made that while bottom-up self-assembly can attain a certain degree of complexity, truly complex systems such as life can only come into being if top-down processes come into play in addition to bottom up processes. They allow genuine emergence to occur, based in multiple realisability at lower levels of higher level structures and functions.