Yeah, that was the divide I was talking about, more or less. Day-to-day skepticism is pretty viable (and an incredible analytic tool, regardless), but I'd sorta' parallel it to (beneficial) lay worship. The skeptical equivalent of fundamentalism being th'one you identified as the harder philosophical skepticism.
I don't agree with either side of that analogy.
Comparing philosophical scepticism to fundamentalism doesn't do either side much credit. There are branches within scepticism that do closely match fundamentalism, but they aren't comparable catagories.
Philosophical scepticism would be more akin to abstract theology. It is a doctrine for exploring the territory (knowledge for scepticism, the varied concepts of God and the divine for theology), not for dictating how you live. It may inform beliefs closer to real life and change your perspective on other ideas, but it doesn't in itself tell you much of anything about how you should act. Even people who are strongly philosophical sceptics are unlikely to live their lives by it's teachings or even find such teachings that inform their lives.
The comparison of rational scepticism to lay worship is stronger, in the sense that they directly inform your day-to-day life, but the fundamental difference is that scepticism can and should be turned on itself. While lay religion
may come with such a doctrine, it is only occasional and far from a guaranteed part of religion.
I wouldn't mention that, but the idea that scepticism is doctrinal in the same manner as much religion comes up a lot and is a really nasty false equivalence. It's reflex to push back hard.
Further away you get from that kind of hardcore request for justification of knowledge claims, th'more you're basically capitulating to usability, as I see it.
Thing is, I don't see that at all. Mostly because I don't think that hardcore requests for justification are what it's about.
Again, scepticism doesn't demand that you have certainty before accepting a belief. Hell, the more hardcore versions dictate that you can never have certainty, and frankly they have a strong point there. It's more about ensuring that beliefs are reasonable and being willing to challenge them.
It's possible to hold a belief while acknowledging it's possible it is wrong. This is practically a requirement for scepticism. It's why the fundamental question isn't, "why is this true," it's, "why do I believe what I believe?" Understanding your reasons for belief helps reveal how reliable that belief is, as well as what other assumptions it rests on.
In this sense anything less than strict philosophical rigour isn't a concession to usability but simply an acceptance of a different level of certainty. An idea that has a complete and valid philosophical/logical chain of reasoning might be as certain as the branch of logic or philosophy the chain is built in. A belief that is based on something some guy said in the pub is as certain as your trust in the guy and your immediate assessment of that information given prior knowledge. It doesn't require you test it before thinking it's true, but does mean you try to understand the reason you believe it to be true and maybe consider it needs more testing or evidence before you take any actions based on it.
When I spoke of human, practical level, this is what I meant. It gives us a level of certainty that people are able to act on. The thing is, this isn't just an appeal to popularity. It's also a reasonably objective statement of that level of certainty (at least from where I'm sitting). We can trust inductive reasoning strongly enough to commit decades of work to building inductive chains of logic, solely on the entirely inductive reasoning that such arbitray reasoning (AKA, science) happens to give good results (eg, technology).
Of course, some stuff is lost to practicality in practice. It's not like certainty levels are quantified or formal, and usually the language of certainty is entirely reasonable to use. But then usually the discussions between sceptics and others take place in territory where there such language is well justified, especially from a sociological point of view. If you are having an argument over the validity of some harmful quack medical practice then you probably want to make as definitive a statement as you can, even if that's a (slightly) exaggerated position from a less definitive position held based on statistical evidence and Bayesian inference.
As an example, I'd say that I disbelieve the Christian God. Not that I don't believe, but actively disbelieve. I base this on my reading of the logical arguments for and against such a deity. Now, none of these are truly definitive arguments. There is no golden bullet that shows that (at least subsets of even that narrow definition of) God doesn't exist. There is the possibility of being wrong. But my reading of the arguments gives me strong enough reason for me to hold that positive belief and act as though it were definite.