“For out of the overflow of the heart the mouth speaks. The good man brings good things out of the good stored up in him, and the evil man brings evil things out of the evil stored up in him” (Matthew 12:34-35).
I would like to draw your attention to the words overflowand stored up. Whatever is stored up is brought out. Good stored up; good comes out. Evil stored up; evil comes out. Not only is good or evil brought out, but they overflow out.
Apparently from this text the mouth is an overflow valve venting what is stored up. The teaching here is not on watching your mouth, but on storing up good. If we endeavor and succeed in keeping our mouths shut to keep the stored-up evil from coming out, it would be like blocking the safety valve on a pressure cooker. Sooner or later there will be an explosion, and the contents would come out anyway. The emphasis is on the heart not on the mouth. Let us store up good. We then will not have to watch our mouths.
Written in 1983.