Applying logic to pithy quotes is generally going to be painful, because there's tends to be a lot of implicit meaning. But let's have some fun and break this down.
(Bear with me, it's been a while since I've done rigorous proofs.)
Let's say there's a function R that outputs a numerical value of an actions morality.
Let's define an action as wrong if R(action) is below some threshold M.
So what Lincoln is saying is:
R(slavery) >= M -> R(action) >= M for all actions
If we assume Lincoln's statement to be true, that must mean that R(slavery) <= R(action) for all actions.
Which can be reduced to R(slavery) = min(R(actions))
Translated to English, Lincoln is saying that slavery is most morally despicable act.
(Bear with me, it's been a while since I've done rigorous proofs.)
Let's say there's a function R that outputs a numerical value of an actions morality.
Let's define an action as wrong if R(action) is below some threshold M.
So what Lincoln is saying is:
R(slavery) >= M -> R(action) >= M for all actions
If we assume Lincoln's statement to be true, that must mean that R(slavery) <= R(action) for all actions.
Which can be reduced to R(slavery) = min(R(actions))
Translated to English, Lincoln is saying that slavery is most morally despicable act.