I approached it similarly (as part of a more general attempt, since this is a minor use of the word), positing the "I could lift that box over there" was a comparison of the physical prowess necessary to complete the task and the amount I currently possess. In Eliezer's formulation, this is equivalent to determining reachability with constraints, but it's more of an example of the general procedure than an explanation of it, unfortunately. I'm glad to see that someone else was thinking similarly though.