The first difficulty many encounter in understanding Simulationism is in recognizing that we don’t have to simulate our Earthly reality. Ars Magica simulates Medieval wizardry, uses Medieval physics, and conforms to a Medieval understanding of reality. Ascendant simulates comic books, uses comic book physics, and is structured to recreate comic book outcomes. Neither applies relativistic physics, for reasons that should be obvious.
But the second difficulty, which follows this one, is more subtle and nuanced. If we can simulate any reality, then are we really simulating anything at all? Aren’t narrativist storygames just a simulation of an appropriate genre, or of a world like that of A Practical Guide to Evil, in which narrative conventions hold real power in-world? They leave thinking simulationism is nonexistant, and appreciating it a result of brain damage. And this is a much more concerning outcome, that demands an answer to the question: what separates a simulationist game in a narrative-conforming world from a narrativist game? (And, though I haven’t actually encountered, the theoretical variant from gamist perspective: what simulates a simulation of a game-conforming world from a gamist RPG?)
The answer is coherence.
A simulationist world is a coherent model of an imagined reality, not merely a vehicle for some story or competition. It follows it’s own rules internal to itself, that are consistent and make sense in-world. To break that down further, three key concepts come to mind: Causality, Experience, and Symmetry.
Causality
Things happen in a simulationist game because they were caused to happen, in-world. There aren’t dissociated mechanics that run on metagame logic without an in-world correlate, which should be clear from a brief examination of such mechanics — they universally serve gamist and narrativist ends. Likewise, simulationism doesn’t lend itself to a lot of retroactive mechanics for this reason, and when it does, they generally exist that way in-world.
Moreover, the progression of in-world events isn’t driven by the needs of plot, nor by attempts to create a challenging environment for adventure (though those needs might be proactively accounted for when creating such a campaign). New developments occur only because in-world actors chose to take action, utilizing finite resources, in advancement of their own goals and desires. Running such characters and seeing the world through their eyes, and shaping it through their hands, is one of the highest pleasures of Judging. If the Judge is busy or seeking yet further depth, such responsibilities might be delegated to patron players. But always, these happenings come about because of in-world action and desires, not to serve the interests of the metagame or Judge (and thus we see the need for Judge neutrality).
Experience
To put it simply, the experience of playing a simulationist game should feel like the thing it simulates. Being an agile striker versus an armored bruiser should be a qualitatively different experience for the player. If using a machinegun lets you throw a whole bunch of dice for your attack, while a sniper only throws one but with a hefty bonus, that distinction at a mechanical level reinforces the simulation and ties the player’s experience to the character’s experience. Similarly, if reaction rolls happen on a normally distributed and strongly centered 2d6 throw, whereas combat rolls where anything might happen occur on 1d20, that again creates a difference of experience that matches the difference in what is occurring.
The rise of universal resolution mechanics undermines this, and is one of the things that really bothers me when I try to play a game like Blades in the Dark — it’s a very well done game, but everything kind of starts to feel the same, because it all follows the same mechanical framework. It was a good choice in 3e D&D to have skills work differently from attack throws, opposite to common opinion, and in fact 3e would have done better to differentiate it even further. This can increase complexity if done excessively, of course, so it pays to have a discriminate hand, but there are also many ways keep such frameworks simple (for example, observe how weapon charts have gradually evolved with standardized keywords and tags).
Symmetry
The rules of a simulationist game are invariant. If one player tries something, another player can reasonably expect that it’ll work the same way for him. Likewise, doing something today and then again a month from now, there won’t be a difference in the method of resolution used. Or yet again: if a PC does something according to certain rules, those rules apply just the same to NPCs as well.
One might protest, for example, that ACKS is a simulationist game and yet gives monsters a different attack progression from PCs. But that’s not a PC-NPC difference — if a PC played a monster (e.g., a Thrassian Gladiator), he too would use the monster attack progression. And the inverse is true as well, inasmuch as a levelled NPC uses the attack progression for their class, not that of a monster. Compare to 5e D&D, where classed and levelled characters have monster statblocks in the Monster Manual with dramatic differences compared to building them as a PC (not to mention that PCs can accumulate Inspiration, which NPCs cannot). Such a world is asymmetric with regards to the PC-NPC distinction.
It’s worth noting that this doesn’t mean PCs can’t be special people with weighty destinies, Fate Points, or similar advantages — but it means that NPCs can and should have access to such powers as well (FATE itself fails in this regard among others by giving the Judge a pool of Fate Points, instead of attaching them to specifically fated characters).
Done properly, this preserves the integrity of the world from metagame constraints or arbitrary variation, and is a valuable component of versimilitude.
Conclusion
When people Experience a world with real Causality, where everything follows a rational Symmetry, they experience noetic appreciation of verisimilitude. They have not suspended their disbelief; they have been shown something real. That is the key to the appreciation of Simulationism writ large. I will close with a quote from J. R. R. Tolkien, who puts it beautifully in his profound essay On Fairy Stories:
That state of mind has been called “willing suspension of disbelief.” But this does not seem to me a good description of what happens. What really happens is that the storymaker proves a successful “sub-creator.” He makes a Secondary World which your mind can enter. Inside it, what he relates is “true”: it accords with the laws of that world. You therefore believe it, while you are, as it were, inside. The moment disbelief arises, the spell is broken; the magic, or rather art, has failed. You are then out in the Primary World again, looking at the little abortive Secondary World from outside. If you are obliged, by kindliness or circumstance, to stay, then disbelief must be suspended (or stifled), otherwise listening and looking would become intolerable. But this suspension of disbelief is a substitute for the genuine thing, a subterfuge we use when condescending to games or make-believe, or when trying (more or less willingly) to find what virtue we can in the work of an art that has for us failed.
A solid explanation of the why and wherefore of, and of the difference between, the overall approaches.