The thought which lies below the surface is that of the inseparable union between truth and justice. The end does not justify the means, and only he who breathes and utters truth makes the righteous cause clear.

He that speaketh truth showeth forth righteousness,.... He that "blows" or "breathes out truth" (d), as the word signifies; that utters it freely and fully without any hesitation; that speaks nothing but truth, and speaks out the whole truth without any reserve; such a man upon every occasion will declare that which is just and right, and show himself to be an honest and upright man; he that uses himself to speak truth in common conversation, will, in a court of judicature, whether upon his oath or not, testify that which is just and the real matter of fact; there is a connection between truth and righteousness, for though they are distinct things they go together, what is true is just, and what is just is true; so he that speaks the truth of the Gospel, or is a faithful preacher of it, will show forth righteousness, what is the righteousness of the law, and what is the righteousness of faith; how insufficient a man's own righteousness is to justify him in the sight of God; the necessity of the righteousness of Christ, how free and full, excellent and glorious, suitable and useful it is, Romans 1:17;
but a false witness deceit; that is, one that is used to lying, when he is called to give testimony upon any affair in judgment, he will declare that which is false and deceitful, having no regard to truth and justice. So a false teacher, instead of showing men the insufficiency of their own righteousness, and directing them to the righteousness of Christ for justification, will utter deceitful doctrine, and build up their hopes upon the sandy and deceitful foundation of their own works; as Popish teachers, and such as verge towards them.
(d) "effiat", Junius & Tremellius; "spirat", Schultens.

It is good for all to dread and detest the sin of lying, and to be governed by honesty.

Most of the remaining parables of this section refer to the right use and the abuse of the tongue.
17 He that breathes the love of truth, utters that which is right;
But a lying tongue, deceit
This verse is similar in meaning to Proverbs 14:5 (where 5b = Proverbs 6:19); the second line of the distich = Proverbs 14:25. Everywhere else יפיח כּזבים stand together, only here יפיח is joined to אמוּנה; vid., regarding this יפיח forming an attributive clause, and then employed as an adjective, but with distinct verbal force, at Proverbs 6:19. Viewed superficially, the proverb appears tautological; it is not so, however, but places in causal connection the internal character of men and their utterances: whoever breathes אמוּנה, truth or conscientiousness (the property of the אמוּן, vid., at Psalm 12:2), i.e., lets the voice of this be heard in his utterances, such an one speaks צדק, i.e., uprightness, integrity, that which is correct, right (Isaiah 45:19, cf. Isaiah 41:26), in relation to truth in general, and to the present case in particular; but he who עד שׁקרים, i.e., he who, against better knowledge and the consciousness of untruth, confirms by his testimony (from עוּד, revertere, to say again and again), therewith gives utterance to his impure character, his wicked intention, proceeding from delight in doing evil or from self-interest, and diverted towards the injury of his neighbour. As אמונה and מרמה correspond as statements of the contents of the utterances, so צדק and שקרים as statements of their motive and aim. מרמה is obj. accus. of the יגּיד (from הגּיד, to bring to light, cf. נגד, visibility) to be supplied, not the pred. nom. dolorum structor, as Fleischer poetically finds.

Deceit - He who uses himself to lying in his common talk, will use falsehood and deceit in judgment.