Please reply in the comments with things you understood recently. The only condition is that they have to be useless in your daily life. For example, "I found this idea that defeats procrastination" doesn't count, because it sounds useful and you might be deluded about its truth. Whereas "I figured out how construction cranes are constructed" qualifies, because you aren't likely to use it and it will stay true tomorrow.
I'll start. Today I understood how Heyting algebras work as a model for intuitionistic logic. The main idea is that you represent sentences as shapes. So you might have two sentences A and B shown as two circles, then "A and B" is their intersection, "A or B" is their union, etc. But "A implies B" doesn't mean one circle lies inside the other, as you might think! Instead it's a shape too, consisting of all points that lie outside A or inside B (or both). There were some other details about closed and open sets, but these didn't cause a problem for me, while "A implies B" made me stumble for some reason. I probably won't use Heyting algebras for anything ever, but it was pretty fun to figure out.
Your turn!
PS: please don't feel pressured to post something super advanced. It's really, honestly okay to post basic things, like why a stream of tap water narrows as it falls, or why the sky is blue (though I don't claim to understand that one :-))
Yeah, I mentioned the topology complications.
How so? I thought removing the border on each negation was the right way. (Also you need to start out with no border, basically you should have open sets at each step.)
Lambda calculus is indeed a nice way to understand intuitionism, that's how I imagined it since forever :-) Also the connection between Peirce's law and call/cc is nice. And the way it prevents confluence is also really nice. This stackoverflow question has probably the best explanation.
I gave an example of where removing the border gives the wrong result. Are you asking why "A is a subset of Not Not A" is true in a Heyting algebra? I think the proof goes like this:
May... (read more)