What is a Simulation?

While reading Paul Rosenbloom’s outstanding book “On Computing” I came across an interesting question: what is a simulation, and how is it different from an implementation? I posed this question on Facebook and, thanks to the superlative quality of my HiveMind I had a productive back-and-forth which helped me nail down a tentative answer. Here it is:

‘Simulation’ is a weaker designation than ‘implementation’. Things without moving parts (like rocks) can be simulated but not implemented. Engines, planets, and minds can be either simulated or implemented.

A simulation needs to lie within a certain band of verisimilitude, being minimally convincing at the lower end but not-quite-an-implementation on the other. An implementation amounts to a preservation of the components, their interactions, and the higher-level processes (in other words: the structure), but in a different medium. Further, implementation is neither process- nor medium-agnostic; not every system can rise to the level of an implementation in any arbitrarily-chosen medium.

A few examples will make this clearer.

Mario is neither a simulation not an implementation of an Italian plumber. If we ran him on the Sunway TaihuLight supercomputer and he could pass a casual version of the Turing test, I’d be prepared to say that he is a simulation of a human, but not an implementation. Were he vastly upgraded, run on a quantum computer, and able to pass as a human indefinitely, I’d say that counts as an implementation, so long as the architecture of his mind was isomorphic to that of an actual human. If it wasn’t, he would be an implementation of a human-level intelligence but not of a human per se.

A digital vehicle counts as a simulation if it behaves like a real vehicle within the approximate physics of the virtual environment. But it can never be an implementation of a vehicle because vehicles must bear a certain kind of relationship to physical reality. There has to be actual rubber on an actual road, metaphorically speaking. But a steam-powered Porsche might count as an implementation of a Porsche if it could be driven like one.

An art auction in the Sims universe would only be a simulation of an art auction, and not a very convincing one. But if the agents involved were at human-level intelligence, that would be an implementation of an art auction. Any replicated art within the virtual world wouldn’t even count as a simulation, and would just be a copy. Original art pieces within a virtual world might count as real art, however, because art doesn’t have the same requirement of being physically instantiated as a vehicle does.

Though we might have simulated rocks in video games, I’m not prepared to say a rock can ever be implemented. There just doesn’t seem to be anything to implement. Building an implementation implies that there is a process which can be transmogrified into a different medium, and, well, rocks just don’t do that much. But you could implement geological processes.

Conway’s Game of Life is only barely a simulation of life; it would probably be more accurate to say it’s the minimum viable system exhibiting life-like properties. But with the addition of a few more rules and more computing power it could become a simulation. It would take vastly more of both for it to ever be an implementation, however.

My friends’ answers differed somewhat from the above, and many correctly pointed out that the relevant definitions will depend somewhat on the context involved. But as of March 29th, 2017, I’m happy with the above and will use it while grappling with issues in AI, computing, and philosophy.

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