The logic tutor is a bit of fun, I'm also learning the boring way, from a book! It's nice to have two sources and two different ways of learning and I think I'm questioning enough not to take any single source as gospel. I agree circular statements can be useful but I'd like to know something is circular - I think I do mostly but there's room to improve.
The Ven diagram stuff is the part I find the hardest, even if I draw out the diagrams I still struggle. I don't struggle too much with 'real world' classification systems it's the abstract ones used to teach that get me in a knot.
When I was teaching autistic children years ago I thought the 'same vs different' programmes and categories were really core stuff. I even mused that maybe intelligence is built on discrimination and grouping together. My foray into formal logic makes me wonder if I was on the money with my musings.