Emil Schürer writes (The Literature of the Jewish People in the Time of Jesus, pp. 329-331):
While this shorter explanation in a catechetical form [Questions and Answers on Genesis] was intended for more extensive circles, Philo’s special and chief scientific work is his large allegorical commentary on Genesis, Νομων ιερων αλληγοριαι (such is the title given it in Euseb. Hist. eccl. ii. 18. 1, and Photius, Bibliotheca cod. 103. Comp. also Origen, Comment. in Matth. vol. xvii. c. 17; contra Celsum, iv. 51). These two works frequently approximate each other as to their contents. For in the Quaestiones et solutiones also, the deeper allegorical significance is given as well as the literal meaning. In the great allegorical commentary on the contrary, the allegorical interpretation exclusively prevails. The deeper allegorical sense of the sacred letter is settled in extensive and prolix discussion, which by reason of the copious adducting of parallel passages often seems to wander from the text. Thus the entire exegetic method, with its draggin in of the most heterogeneous passages in elucidation of the idea supposed to exist in the text, forcibly recalls the method of Rabbinical Midrash. This allegorical interpretation however has with all its arbitrariness, its rules and laws, the allegorical meaning as once settled for certain persons, objects and events being afterwards adhered to with tolerable consistency. Especially is it a fundamental thought, from which the exposition is everywhere deduced, that the history of mankind as related in Genesis is in reality nothing else than a system of psychology and ethic. The different individuals, who here make their appearance, denote the different states of soul (τροποι της ψυχης) which occur among men. To analyse these in their variety and their relations both to each other and to the Deity and the world of sense, and thence to deduce moral doctrines, is the special aim of this great allegorical commentary. Thus we perceive at the same time, that Philo’s chief interest is not—as might from the whole plan of his system be supposed—speculative theology for its own sake, but on the contrary psychology and ethic. To judge from his ultimate purpose he is not a speculative theologian, but a psychologist and moralist (comp. note 183).

The commentary at first follows the text of Genesis verse by verse. Afterwards single sections are selected, and some of them so fully treated, as to grow into regular monographs. Thus e.g. Philo takes occasion from the history of Noah to write two books on drunkenness (περι μεθης), which he does with such thoroughness, that a collection of the opinions of other philosophers on this subject filled the first of these lost books (Mangey, i. 357).
The work, as we have it, begins at Gen. ii. 1; Και ετελεσθησαν οι ουρανοι και η γη. The creation of the world is therefore not treated of. For the composition, De opificio mundi, which precedes it in our editions, is a work of an entirely different character, being no allegorical commentary on the history of the creation, but a statement of that history itself. Nor does the first book of the Legum allegoriae by any means join on to the work De opificio mundi; for the former begins at Gen. ii. 1, while in De opif. mundi, the creation of man also, according to Gen. ii, is already dealt with. Hence—as Gfrörer rightly asserts in answer to Dähne—the allegorical commentary cannot be combined with De opif. mundi as though the two were but parts of the same work. At most may the question be raised, whether Philo did not also write an allegorical commentary on Gen. i. This is however improbable. For the allegorical commentary proposes to treat of the history of mankind, and this does not begin till Gen. ii. 1. Nor need the abrupt commencement of Leg. alleg. i seem strange, since this manner of starting at once with the text to be expounded, quite corresponds with the method of Rabbinical Midrash. The later books too of Philo’s own commentary begin in fact in the same abrupt manner. In our manuscripts and editions only the first books bear the title belonging to the whole work, Νομων ιερων αλληγοριαι. All the later books have special titles, a circumstance which gives the appearance of their being independent works. In truth however all that is contained in Mangey’s first vol.—viz. the works which here follow—belongs to the book in question (with the sole exception of De opificio mundi).
Emil Schürer comments:
“Περι γιγαντων. De gigantibus (Mangey, i. 262-272). On Gen. vi. 1-4. Οτι ατρεπτον το θειον. Quod deus sit immutabilis (Mangey, i. 272-299). On Gen. vi. 4-12. These two paragraphs, which are in our editions separated, form together but one book. Hence Johannes Monachus ineditus cites passages from the latter paragraph with the formula εκ του περι γιγαντων (Mangey, i. 262, note, 272, note). Euseb. H. E. ii. 18. 4: περι γιγαντων η [elsewhere και] περι του μη τρεπεσθαι το θειον.” (The Literature of the Jewish People in the Time of Jesus, pp. 334-335)
J. H. A. Hart writes (The Jewish Quarterly Review Original Series 17, pp. 95-97):
The division between the tracts Concerning Giants and That the Divine is unchangeable seems hardly warranted, as the former ends with the words “Having said thus much—sufficient for the present at any rate—concerning the giants, let us turn to the sequel of the narrative. And it is this.” It is not uncommon to find two different subjects treated in the same tract (cf. e. g. Concerning the Progeny of Cain, etc.).

The “many men” of Gen. vi. 1 are obviously impious men, because their children are daughters. The story of the union of these daughters with the angels of God is not a myth. Just as the universe is animated (εψυχωσθαι) throughout all its parts, earth, water, fire (especially, it is reported, in Macedonia) and heaven (with stars), so the air must be filled with living things, invisible to us like the element in which they live. What Moses calls angels other philosophers call demons, souls flying about in the air. Surely air which gives life to all creatures has a natural right to a population of its own. Well, then, some souls have descended into bodies and some of them are able to resist the current of human life and fly up again: these are the souls of true philosophers, who from beginning to end practise dying to bodily life (βιου) that they may share the bodiless and incorruptible life (ζωης). Other souls, again, disdained union with any part of earth, and these hallowed souls, who are concerned with the service of the Father, the Creator is wont to use as servants and ministers for the protection (επιστασιαν) of mortals. These are of course the good angels, angels worthy of the name. There are bad angels also, of whom the many speak as bad demons or souls, and it is they who descended to converse with the daughters of men.

Here Philo is once more in agreement with the Stoics, who held that the souls of the dead (or of the righteous dead) existed in the air until the great conflagration in which the universe was to be consumed, and that there were also demons sympathetic with men, watchers (εποπτας) of human affairs (Diog. vii. 151, 156, 157). The statement that the universe is alive (εμψυχον) and full of demons is attributed to Thales and Heraclitus. Philo expounds again his doctrine of demons or angels in de Somn. i. §§ 134ff. in connexion with Jacob’s dream of a ladder reaching from earth to heaven. The body he regards, with Plato, as a prison or tomb, and the purest and best souls or spirits are those which never yearned for earthly life, the proconsuls of the All-ruler, who correspond to the lesser deities with whom Plato surrounds the Creator (Tim. 41 A).

But in evil men God’s spirit cannot remain permanently (ου καταμενει, Gen. vi. 3). It remains indeed on occasion “For who is so devoid of reason or soul as never, willing or unwilling, of his own will or without, to receive a conception of the Best? Nay, indeed, even upon the accursed there alights often of a sudden the appearance of the Good (του καλου), but they cannot appropriate it or keep it with themselves. For it departs, removing straightway, renouncing the stranger in the land who has forsaken (εκδεδιητημενους) law and right, to whom it would never have come at all save to convict them as having chosen base things instead of honourable.”
Such men are flesh; and the fleshy nature is the foundation of ignorance. But the Law, in the ordinance against unlawful unions, commands us to despise the flesh (Lev. xviii. 6). A man that is truly a man—such an one as one of the ancients (Diogenes the Cynic) sought with lighted lantern at noon—will not approach that which belongs to his flesh. The emphatic repetition of the word man in the (Greek) text of the passage shows that it is not the ordinary human being but the virtuous man who is meant (ανθρωπος ανθρωπος προς παντα οικειον σαρκος αυτου ου προσελευσεται). They who fail to keep this law degrade themselves, “reveal their unseemliness”; and such are the self-styled wise who sell wisdom and cheapen their wares like cheapjacks in the market.

The giants who issue from this union are not those of Greek mythology: “Moses wishes to impress upon you that some are men of earth, others men of heaven, and others men of God. The men of earth are the hunters of bodily pleasures, who practise the use and enjoyment thereof and provide whatever contributes to each one of them. The men of heaven are all artists, craftsmen and scholars; for the heavenly part of ourselves—the mind—practises general education, and the other arts, one and all, sharpening and whetting, exercising and training itself in the ideal things (τοις νοητοις). The men of God are priests and prophets who disdained any state connected with this world . . . and have emigrated to the ideal world where they dwell, enrolled in the state of incorruptible and bodiless ideas.” For example, Abram, “lofty father,” is a man of heaven and rises to become Abraham “elect father of sound,” that is a man of God (Gen. xvii. 1). Whereas the children of earth, like Nebrod (Gen. x. 8), are deserters degraded from their proper rank to the lifeless and motionless nature of flesh, as it is written “they twain shall be one flesh” (Gen. ii. 24).
F. H. Colson and G. H. Whitaker write (Philo, vol. 2, pp. 443-445):
This short, but in many ways beautiful and more than usually Platonic treatise, is very closely connected, as the last words show, with the succeeding “Quod Deus,” which will follow in Vol. III. of this translation. It is a dissertation on the words of Gen. vi. 1-4.

(a) And it came to pass when men began to become many upon the earth that daughters were born to them. (1)

(b) And the angels of God, seeing the daughters of men that they were fair, took to themselves wives from all, such as they chose. (2)

(c) And the Lord God said, “My spirit shall not abide in man for ever, because they are flesh; but their days shall be a hundred and twenty years.” (3)

(d) And there were giants on earth in those days. (4)

(a) is dismissed shortly (1-5) with the remarks that the words “many” and “daughters” following on the mention of the birth of Noah, the just man, and his three sons (at the end of chap, v.) emphasize the truth that the unjust are many and the just few, and that the spiritual offspring of the latter are the masculine or higher qualities, while that of the former are the feminine or lower.

(b) The words are interpreted (6-18) in the sense that as angels, demons, and souls are really three names for the same thing, “the angels of God,” while including God’s spiritual messengers, here indicate the wicked souls which woo the “daughters of men,” i.e. the merely sensual pleasures. In the course of these sections we have a remarkable passage (12-15), in which Philo, with many echoes of Plato, speaks of the human soul as having descended from some higher region to be incarnate in the body.
The discussion of (c) (19-57) forms the bulk of the treatise. He first treats (19-27) of the nature of God’s spirit, dwelling particularly on the thought that when it is given to men, it is not thereby diminished, and on the unworthiness of the fleshly life (28-31). This leads him on to a long digression on Lev. xviii. 6, “a man, a man shall not go near to any that is akin to his flesh, to uncover shame.” This text, which of course is really a prohibition of incest, is worked by Philo into an elaborate allegory, in which every phrase is treated separately (32-47). The repetition of “a man, a man” indicates the “true man” (33). The words “go near” show us that while many earthly advantages, such as riches, though “akin to the flesh,” must be accepted, if they come to us, and used for the best, we must not seek them (84-38). “Uncovering shame” means (39) that those who follow such things introduce a false and shameful philosophy. The final words, “I am the Lord,” are an appeal to us to take our stand with God against pleasure (40-44), but the use of “Lord” rather than “God” emphasizes his attitude of sovereignty of which we are bidden to stand in awe (45-47). We now return to the thought of what is meant by God’s spirit abiding. Such an abiding can only be the privilege of those who lead the tranquil and contemplative life, which with the support of various texts he ascribes to Moses (47-55). The words “their days shall be an hundred and twenty years” are then touched on for a few lines (56), but dismissed with a promise of subsequent treatment, which if ever given has not come down to us (57).

(d) After a protest against regarding the story as a myth (58-59), we have a meditation (60-67) on the three classes of souls, the earth-born (who of course are the giants, γιγας being connected with γηγενης), the heaven-born, and the God-born. Of these the heaven-born are those who cultivate our heavenly part, the mind, and follow secular learning (60), and the God-born are those whose thoughts are fixed on God alone (61). These two are illustrated by Abram (before his change of name) and Abraham respectively (62-64). The earth-born, of course, are those who are given up to the fleshly life, and are typified by Nimrod (who in the LXX is called a giant) whose name signifying “desertion” marks the earth-born “giants” as deserters from the good (65-67).