How Do We Fight What We Can’t See?

Ever since we were kids we were told that there were no monsters under our bed, or beasts in our closets. That the black figure suspended in midair slowly floating towards us is only a play of shadows all created by our imagination. We were told that it was all only in our heads as if it was supposed to make us feel any better, as if knowing this could somehow erase our fears. They were wrong.
For how do we fight what we can’t see?