You don't, that's why I made those qualifications in the post. But we can't be paralyzed by inaction over the fact that our knowledge cannot ever be perfect. We have to do the best we can, and create the best policies we can, with the information at our disposal.
Philosophy takes care of ensuring that "best we can" part, but it can't guarantee the "information at our disposal" part.

Cows can fly
Jersey's are cows.
Therefore Jersey's can fly.
Perfectly logical - but the conclusion is false because a premise was false.

Bats can fly.
Bats are mammals.
Therefore all mammals can fly.
In this case both premises are true - but the conclusion is false, because I didn't follow the laws of logic properly. Specifically I am guilty of the composition/division fallacy - assuming that what's true of part of a set must be true of the whole set. If I remove the word 'all' then this is both logical and true - mammals can indeed fly (even if most can't).