Theophylact of Bulgaria
This too is said to the same purpose: the tongue must not be moved at random, but directed toward what is better.
As we restrain the impulses of a horse by a bridle and change the course of a ship by a rudder, so we must direct the tongue toward what is better.
The expression "so also the tongue" (Jas 3:5) means precisely that the tongue must be directed by sound reason.
Though it is small, it does great things. It kindles a great fire for us, because it too is a fire.
What else does it do? It adorns unrighteousness with eloquence and grace. It defiles the body by persuading women with enticements.
Through temptation it commits murders. Through perjuries it appropriates what belongs to another. It sets on fire the course of Gehenna, being itself set on fire by Gehenna.
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This is seen in the rich man suffering in the flame (Lk 16:24). Thus the tongue, scorching the course of life, defiles life.
Life is a course in that it turns within itself. "So also" means that the tongue too should move in the same way, under rational direction.
It is a small member, yet it does much, both evil and good. "Boasts great things" is said instead of "produces great deeds."