One reason the low cost of carbon offsets might not make it feel okay to fly is if you're trying to think about what behaviors and habits would still be acceptable in a society that is already functioning carbon-neutrally. My intuition is that as regulations become stricter and greenhouse-gas-reducing projects need less crowdfunding, carbon offset prices will rise until they equal the cost of capturing and sequestering the CO2, which is on the order of several hundred dollars per tonne. So it's hard to imagine a future in which flying is still okay at prices even close to what they are today.
Does mean the set of propositions in that are true in world ? In other words, does equal ?
Or does the notion of which propositions are possible to know also depend on which world you're in? (Could it be that and but ?)
There's some recent research by Kevin Dorst and Matthew Mandelkern into the idea that people fall for the conjunction fallacy because they are so often trying to strike a balance between being correct and being informative.
Roughly, the idea is that guessing "Linda is an activist and a bank teller" is so informative, it's sometimes more preferable as a guess than just guessing that Linda is a bank teller. Giving not just true but informative guesses is such an ingrained habit that it's hard to stop and select the option most likely to be true.
You can download their paper here: https://philpapers.org/rec/DORGG or read a blog post by Kevin Dorst here: https://www.kevindorst.com/stranger_apologies/the-conjunction-fallacy-take-a-guess