A Shepherd was about to lock a Wolf up for the night with the sheep. His Dog queried him about the reasonableness of doing that; a wise question.
Be watchful or all will be lost.
Townsend version
A shepherd penning his sheep in the fold for the night was about to shut up a wolf with them, when his Dog perceiving the wolf said, “Master, how can you expect the sheep to be safe if you admit a wolf into the fold?’
Canis, Lupus, et Pastor
Intus septa quidam vesperi congregans oves, fulvum una lupum erat inclusurus. Quo conspecto, canis pastori “Quomodo festinas,” dixit, “oves servare, qui istum ad nos intromittis?”
Perry #365