As we see our own face when we look on the mirror-like surface of the water, so in every heart of man we may see our own likeness. In spite of all diversities we come upon the common human nature in which we all alike share. Others see in the reference to the reflection in the water the thought that we judge of others by ourselves, find them faithful or the reverse, as we ourselves are.

As in water face answereth to face - All men's hearts are pretty nearly alike; water is not more like to water, than one heart is to another. Or, as a man sees his face perfectly reflected by the water, when looking into it; so the wise and penetrating man sees generally what is in the heart of another by considering the general tenor of his words and actions.
"Surely, if each man saw another's heart
There would be no commerce;All would disperse,And live apart."
Hebert.

As in water face [answereth] to face, (h) so the heart of man to man.
(h) There is no difference between men by nature, only the grace of God makes the difference.

As in water face answereth to face,.... As water is as a looking glass, in which a man may behold his own face and another's; or as the face in the water answers to the face of a man, and there is a great likeness between them. All things through water appear greater, as Seneca (m) observes, and so more clear and plain;
so the heart of man to man; one man's heart may be seen and discerned in some measure by another, as by his countenance; for though, as the poet (n) says, "frontis nulla fides", yet the countenance is often the index of the mind, though not an infallible one; wrath and anger in the breast may be seen in the face, as were in Cain's; thus Jacob saw some resentment at him in the mind of Laban, and judged he had some design of mischief against him by the change of his countenance; also what is in the heart of man is discerned by what comes out of it, by his words, and also by his actions; yea, a man may know in a good measure what is in another man's heart, by what he finds in his own: the word of God is a glass, or medium of vision, and like water, in which a man's face is seen, through which a man sees his own heart; the law is a glass, in which an enlightened person sees not only the perfections of God, the nature of righteousness, but also his sin, and the sinfulness of it; this glass mother magnifies nor multiplies his sins, but sets them in a true light before him, by which he discerns heart sins, and sees and knows the plague of his heart; and the Gospel is a glass, wherein he beholds the glory of Christ, sees and can discern whether Christ is formed in him, and he has the grace of the Spirit of God wrought in his soul, as faith, hope, love, repentance, humility, self-denial, &c. moreover, as the face seen in the water is similar to a man's face, so the hearts of men are alike, not merely in a natural sense, see Psalm 33:15; but in a moral and spiritual sense the hearts of unregenerate men are alike, and answer to each other; for they are all equally corrupted, one and depraved; the heart of every man is desperately wicked; the imaginations of the thoughts of the hearts or wicked men, one and all of them, are only evil, and that continually; their affections are inordinately the same, they love and hate the same persons and things; their minds and consciences are all defiled; their understandings are darkened; their wills are averse to that which is good, and bent on that which is evil: and so the hearts of good men are alike; they have all one heart and one way given them; their experiences agree as to the work of grace and conversion; they are all made sensible of sin, the evil of it, and danger by it; they are all brought off of their own righteousness, and are led to Christ to depend on him alone for righteousness, pardon, and eternal life; they are partakers of the same promises in the Gospel, and have the same enemies to grapple with, and the same temptations, trials, and exercises from sin, Satan, and the world; and they have the same things put into their hearts, the laws of God, the doctrines of Christ, and the several graces of the Spirit of Christ; so that there cannot be a greater likeness between a man's face and that seen in the water, than there is between the heart of one saint and another; the hearts of Old and New Testament saints, and of all in all ages and places, answer to one another. The Targum paraphrases it to a sense quite the reverse,
"as waters and as faces which are not like one to another, so the hearts of the children of men are not like one to another;''
and to the same sense are the Septuagint, Syriac, and Arabic versions.
(m) Nat. Quaest. l. 1. c. 6. (n) Juvenal. Satyr. 2. v. 8.

One corrupt heart is like another; so are sanctified hearts: the former bear the same image of the earthly, the latter the same image of the heavenly. Let us carefully watch our own hearts, comparing them with the word of God.

We may see our characters in the developed tempers of others.

19 As it is with water, face correspondeth to face,
So also the heart of man to man.
Thus the traditional text is to be translated; for on the supposition that כּמּים must be used for כּבמּים, yet it might not be translated: as in waters face corresponds to face (Jerome: quomodo in aquis resplendent vultus respicientium), because כּ (instar) is always only a prep. and never conj. subordinating to itself a whole sentence (vid., under Psalm 38:14). But whether כּמּים, "like water," may be an abridgment of a sentence: "like as it is with water," is a question, and the translation of the lxx (Syr., Targ., Arab.), ὥσπερ οὐχ ὅμοια πρόσωπα προσώποις, κ.τ.λ., appears, according to Bttcher's ingenious conjecture, to have supposed כאשר במים, from which the lxx derived כּאין דּמים, sicut non pares. The thought is beautiful: as in the water-mirror each one beholds his own face (Luther: der Scheme = the shadow), so out of the heart of another each sees his own heart, i.e., he finds in another the dispositions and feelings of his own heart (Fleischer) - the face finds in water its reflection, and the heart of a man finds in man its echo; men are ὁμοιοπαθεῖς, and it is a fortunate thing that their heart is capable of the same sympathetic feelings, so that one can pour into the heart of another that which fills and moves his own heart, and can there find agreement with it, and a re-echo. The expression with ל is extensive: one corresponds to another, one belongs to another, is adapted to the other, turns to the other, so that the thought may be rendered in manifold ways: the divinely-ordained mutual relationship is always the ground-thought. This is wholly obliterated by Hitzig's conjecture כּמוּם, "what a mole on the face is to the face, that is man's heart to man," i.e., the heart is the dark spot in man, his partie honteuse. But the Scripture nowhere speaks of the human heart after this manner, at least the Book of Proverbs, in which לב frequently means directly the understanding. Far more intelligible and consistent is the conjecture of Mendel Stern, to which Abrahamsohn drew my attention: כּמּים הפּנים לפנים, like water (viz., flowing water), which directs its course always forward, thus (is turned) the heart of man to man. This conjecture removes the syntactic harshness of the first member without changing the letters, and illustrates by a beautiful and excellent figure the natural impulse moving man to man. It appears, however, to us, in view of the lxx, more probable that כּמּים is abbreviated from the original כאשׁר במים (cf. Proverbs 24:29).

So - So one man resembles another in the corruption of his nature.