It's not like Go where you can actually count the number of reachable positions. Deciding whether a single chess position is reachable can be really hard. Ther

It's not like Go where you can actually count the number of reachable positions. Deciding whether a single chess position is reachable can be really hard. There is a whole class of so called "retrograde" chess problems focussed on that. For example, is the following position reachable? White: Kc3 Ba4 Black: Kd1 Rb5 Bd5