Compare the marginal reference. With this warning the long introduction closes, and the collection of separate proverbs begins. Wisdom and Folly have each spoken; the issues of each have been painted in life-like hues. The learner is left to choose.

But he knoweth not that the dead are there - See on Proverbs 2:18 (note). He does not know that it was in this way the first apostates from God and truth walked. רפאים rephaim; gigantev, the Giants - Septuagint. The sons of men, the earth-born, to distinguish them from the sons of God, those who were born from above. See the notes on Genesis 6:1, etc.
Her guests are in the depths of hell - Those who have been drawn out of the way of understanding by profligacy have in general lost their lives, if not their souls, by their folly. The Septuagint, Syriac, and Arabic make a long addition to this verse: "But draw thou back, that thou mayest not die in this place; neither fix thy eyes upon her; so shalt thou pass by those strange waters. But abstain thou from strange waters, and drink not of another's fountain, that thou mayest live a long time, and that years may be added to thy life." Of this addition there is nothing in the Hebrew, the Chaldee, or the Vulgate, as now printed: but in the editio princeps are the following words: Qui enim applicabitur illi descendet ad inferos; nam qui abscesserit ab ea salvabitur. These words were in the copy from which my old MS. Bible has been made, as the following version proves: Who forsoth schal ben joyned to hir, schal falle doun on to hell: for whi he that goth awai fro hir, schal be saved. Three of my own MSS. have the same reading.

But he knoweth not that the dead are there,.... In the house of this foolish and wicked woman, into which she invites passengers to turn; the simple, that is persuaded by her, does not consider that there are none there to be his companions, but such who are dead in a moral or spiritual sense; that, though they live in pleasure, they are dead while they live. Aben Ezra refers this to "hell" in the next clause; where her guests are, and where those that are slain by her have their everlasting abode; and where the giants are, as the Vulgate Latin and Syriac versions; or the mighty ones she has cast down there, as the Targum; so the word "rephaim" sometimes signifies: and some think that reference is had to the giants of the old world, that corrupted their way on earth, and brought a deluge on it; see Proverbs 7:26;
and that her guests are in the depths of hell; not only in the way to it, and on the brink of it, but in the very midst of it: there are many in hell she has invited into her house, and persuaded to turn in there, and commit fornication with her; and all that worship the beast, or commit spiritual adultery with the whore of Rome, will go down to perdition with her, and have their portion in hell fire, in the lake which burns with fire and brimstone; which they do not consider that are drawn into her idolatrous practices, Revelation 14:9.

(Compare Proverbs 2:18-19; Proverbs 7:27).

He goes within:
18 And he knows not that the dead are there;
In the depths of Hades, her guests.
How near to one another the house of the adulteress and Hades are, so that a man passes through the one into the other, is already stated in Proverbs 2:18; Proverbs 7:27. Here, in the concluding words of the introduction to the Book of Proverbs, addressed to youth, and for the most part containing warning against sinful pleasure, these two further declarations are advanced: the company assembled in the house of lewdness consists of רפאים, i.e., (cf. p. 83) the old, worn-out, who are only in appearance living, who have gone down to the seeming life of the shadowy existence of the kingdom of the dead; her (כסילות) invited ones (cf. Proverbs 7:26, her slaughtered ones) are in the depths of Hades (not in the valleys, as Umbreit, Lwenstein, and Ewald translate, but in the depths, Aquila, Symmachus, Theodotion, ἐπὶ τοῖς βαθέσι; for עמקי is not only plur. to עמק, but also per metaplasmum to עמק, Proverbs 25:3, as אמרי to אמר), thus in שׁאול תּחתּית (Deuteronomy 32:22); they have forsaken the fellowship of the life and of the love of God, and have sunk into the deepest destruction. The house of infamy into which Folly allures does not only lead to hell, it is hell itself; and they who permit themselves to be thus befooled are like wandering corpses, and already on this side of death are in the realm of wrath and of the curse.
(Note: The lxx has considerable additions introduced after Proverbs 9:18, as also after Proverbs 9:12, of which we shall elsewhere speak.)