The Verdict Of History
|The argument I'm objecting to goes "We should do X, because people in the future will see X as obviously right."|
One line of objection to that argument is to ask whether people in the future will in fact see X as obviously right. The (usually implicit) argument in favor of that happening is that history is trending in a certain direction, and X lies in that direction. The argument that history is trending in a certain direction is, in turn, based on the idea that policies that are right eventually win out. That means that we have to first have an argument that policy X is right: X is right, therefore X will be accepted in the future, therefore we should do X. But in that case there's no need for the middle term -- you could reason: X is right, therefore we should do X.
If you don't start with "X is right," then you need another argument for why 1) X will be accepted in the future, and 2) why I should care whether people in the future approve of my actions.
A related objection can be made if the future's knowledge of X's rightness is based on new information that they will have access too -- for example, they'll have experience of whether a certain policy is successful or a failure (e.g. Massachusetts and California will see that the sky still hasn't fallen after a decade of letting same-sex couples marry). The problem here is that, on the one hand, if we can be certain in the present which way that data will turn out, then we don't need to launder that certainty (or rather, the reasons for it) through hypothesized future events before we can use it in the argument. On the other hand, if we can't be certain how events will turn out, they can't be a reason for picking one side or the other of the present argument. (This is not to say you can never express confidence that events will prove you right -- just that those events cannot provide an independent reason for someone today to change their mind to agree with you.)
Veridct of history arguments are comforting (most of us want to believe that justice will prevail and society will come to see that we were right). And they may be rhetorically effective because they play on others' desire for social conformity. But they don't provide a logical justification for any position.