I figured I'd complicate this one step further. "A unless B" can also be symbolized as "A or B" which can be easily proved by using a truth table. assume A-->B symbolized as "~B-->A" "OR AS" "A or B"truth table: (sub A for variable x, and B for y, "v" symbolizes the "or") (~Y-->X) (X v Y) x y t t f T t t T t t f t T t t T f f t f T f f T t f f t F f f F fIf you look at the middle column under each stmt. (the cap letters) you can see that they form a tautology. (a fancy word for logically equivilant). You will end up with the exact same result using either symbolization for every possibility. Another way to see this, possibly a bit easier, is to note that: "~B-->A" is the same as "B v A" (or "A v B" however you look at it b/c) This is evident from the rule of conditional exchange.For anyone who has been through logic this is quite trivial, however if this dosent' make sense to you, or if it's been a while since you've seen logic, just take my word for it. "A unless B", can also be stated as "A or B" (or if not B than A)

A unless B is certainly not the same as B unless A

Kwertee is right. A unless B = ~B -> A (this is the classic indisputable interpretation)B unless A = ~A -> B~B -> A and ~A -> B are the contraposative of one another, and therefore equivalent.