Stephen L. Carter, the Yale Law School professor and novelist, offers a typology for understanding various political ideas, drawing on a schema devised by physicist Michio Kaku:
Class I would include things that seem within the capacity of our politics, even though we have little idea so far how to achieve them. Class II would be those things at the outer edge of political possibility, not likely to come about for many decades. … Class III would include those things that are not possible within our politics as understood. In this category would be, among other things, those that are clearly unconstitutional.
There’s impossible, and then there is impossible.