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I don't think anyone's trying to argue that the human brain isn't enormously complex (unless you just define complexity by the instructions themselves -- people argue about various definitions of complexity). The point is that one can be built with relatively simple iterative instructions. So if a cell which is custom made to work with the particular chemistry of DNA efficiently does the work biologically, or humans and machines do analogous work electronically (maybe not so efficiently), the process should sill be comprehensible and iterative.