Attempt at definition.If I have less slack in my belief system, that means I have more constraints in what counts as ‘evidence’ for a given statement or more preconceptions about what can count as ‘true’ or ‘real’. Either, I can be looking for specific signs/evidence/proofs/data (“I will only consider X if you can prove Y.” “I will only consider X if you show me a person who flosses with their shoelace.”). Or, I can be looking for certain categories or classes of evidence (“I will only consider X if there are studies showing X.” “I will only consider X if your arguments takes a certain form.” “I will only consider X if 5 experts agree.” Etc.) Sometimes, it’s better to have less slack. It makes sense for certain fields of mathematics to have very little slack. Other times, it hinders progress.
Are you trying to define Slack in Your Belief System as “this is what those words together naturally mean” or are you defining it as “this is a useful concept to think about and this is what I choose to name it”?
Before reading your take, I thought about what ‘slack in your belief system’ would mean to me, and I came up with a lot of different things it could mean. Mostly my System-1 response was that SIYBS links back into Anna’s concept that flinching away from truth is about protecting the epistomology: What beliefs could you change without changing everything? What must you defend lest huge chains of logic unravel and mindsets shift in disruptive ways? But also the simple, ‘how tightly am I holding onto these beliefs’ type of thing, a kind of uncertainty, how much you would update on new evidence in an area at all. That does go hand in hand with what types of evidence you’d update on. Often I think people have places where the ‘wrong’ kind of evidence is allowed, because there isn’t ‘right’ evidence that needs to be dislodged, so you have more slack in those places, and so on. Kind of a levels-of-evidence thing. Also could be thought of as how well your system can adjust to counterfactuals or fake frameworks or ad argumentos, and still give reasonable answers.
I do think there’s a specific kind of lack of Slack where you decide that something is Scientific and therefore you can only change your beliefs based on Proper Scientific Studies, and that this is very easy to take too far (evidence is and always is evidence). What this is really saying is that your prior is actually damn close to 0 or 1 at this point, so other types of evidence aren’t likely to cut it, and/or that you think people are trying to trick you so you have to disregard such evidence?
Anyway, it’s certainly an interesting thing to think about, and this is already pretty rambly, so I’ll stop here.