Foxes sometimes in search of food would enter into the grape orchards and devour the grapes and spoil the crop. However, the little foxes were too small to reach the grape bunches so they would chew on the vines and it would kill the whole vine. Instead of the farmer just losing his crop, he would lose his vine which was more disastrous. Spiritually some things we do or allow that we might think are little or insignificant can also be disastrous for us.
Every man is born with a conscience.
It is a gift of God to mankind. However, man can quench his conscience by continually ignoring its voice over a period of time. The result is that his heart becomes hardened and seared so that it no longer speaks to him. Little by little. God provided a conscience even in unregenerate men to point them toward His principles and ways.