Other posters seem to be attacking this from the angle of providing examples of non-turing-complete systems which can describe or 'imagine' turing-complete syst

Other posters seem to be attacking this from the angle of providing examples of non-turing-complete systems which can describe or 'imagine' turing-complete systems. I would attack this by claiming that we have designed turing complete systems using no external input. That is to say - we can _produce_ a turing complete system, which implies we are a turing complete system given an IO port (limbs, for example). Fundamentally I could sit and perform calculations with pen and paper. I'm ignoring the unlimited criterion here, I assume colloquial usage. The only real way I could fathom otherwise is by deciding to isolate the brain and consider it as a single entity. But in that case a CPU isn't really turing-complete because it can't do anything on its' own.