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 του εξενηψε Νωε. De sobrietate (Mangey, i. 392-403). On Gen. ix. 24.—In the best manuscripts (Vaticanus and Mediceus) the title runs: περι ων ανανηψας ο νους ευχεται και καταραται. Hieronymous, vir. illustr. 11: de his quae sensu precamur et detestamur.” (The Literature of the Jewish People in the Time of Jesus, p. 335)

J. H. A. Hart writes (The Jewish Quarterly Review Original Series 17, pp. 116-118):

The de Sobrietate naturally follows the de Ebrietate (though the latter is perhaps imperfect, lacking as it does any full exposition of the nakedness of Noah), and the discourse deals with Gen. ix. 24-27. Philo has little to say about sobriety, but that nothing can be better than a sober intellect, nothing so valuable as the clear insight of the soul which it brings. This done, he turns to the text and fastens on “the younger son,” which is proved from Scripture parallels to refer not to age but to maturity of mind. Ishmael, the sophist, though a youth, is called a child in comparison with Isaac the philosopher (Gen. xxi. 14-16). The whole people Scripture calls children (Deut. xxxii. 4-6) when they behave as such. Rachel, who stands for bodily beauty, is younger than Leah the beauty of the soul. Joseph is always young or younger (Gen. xxxvii. 2; xlix. 22). Similarly, elder is first applied to the wise Abraham, the shortest-lived of all the patriarchs (Gen. xxiv. 1). The seventy colleagues of Moses are elders whom the wise man knows (Num. xi. 16). The significance of these terms is clearly set forth, for those who are skilled to hear, in one commandment of the Law, viz. that relating to the children of the beloved and hated wives (Deut. xxi. 15-17). The beloved wife is the symbol of pleasure, her child the pleasure-loving temper; the hated wife is the symbol of understanding, and her child the love of virtue. The first is always a child, the second an elder from his cradle. Accordingly Esau, the elder in point of age, resigns his birthright to Jacob; and Ephraim, who is “Fruitfulness,” i. e. Memory, is preferred before Manasseh, who is Forgetfulness.

But why does Noah curse the child of the offender and not the offender himself (Gen. ix. 25)? Wherein did Canaan sin? Well, those who are accustomed to elaborate the literal and superficial meanings contained in the laws have considered them by themselves perhaps, but let us obey the suggestions of right reason and interpret the underlying meaning. Ham means “hot,” Canaan “commotion.” Both are evil, the one quiescent, the other in motion. Rightly then is Canaan the son of Ham, and rightly is Canaan cursed. For being moved to sin Ham himself becomes Canaan. So is the law that the sins of the fathers are visited on the children (Ex. xx. 5) justified; the results, or children, of reasonings are punished, while they, if no culpable action be laid at their door, escape accusation.

Shem is, as has been said before, the eponymous good kind of man, and God is his God. He who, like Shem and Abraham (Gen. xviii. 7), has God as his portion (κληρον) has passed beyond the bounds of human happiness.

With regard to the blessing of Japheth, we are not clearly told who is to dwell in the tents of Shem. It is possible to understand that it is the Lord of the universe. What more fitting home could be found for God than a soul perfectly cleansed, counting virtue (το καλον) the only good? Of course he will dwell there not as in a place—contained therein—but as bestowing special forethought and attention upon it, like every master of a house. But perhaps the whole prayer refers to Japheth, that he may reckon all worldly goods at their true rate and seek only those of the soul.

F. H. Colson and G. H. Whitaker write (Philo, vol. 3, pp. 438-441):

In this short treatise Philo concludes his discussion of Gen. ix. 20-27, which describe Noah’s husbandry, vine-planting, drinking the wine, intoxication and nakedness, return to sobriety, and cursing or blessing his children. The verses here treated (24-27) run as follows:

I. (sections 1-20 of this treatise) And Noah returned to soberness from the wine and knew what his younger son had done to him.

II. (30-50) And he said, “Cursed be Canaan; a servant and bondman shall he be to his brethren.”

III. (51-58) And he said, “Blessed be the Lord God of Shem; and Canaan shall be a servant, a bondman of him.”

IV. (59-end) And he said, “May God widen for Japhet, and let him dwell in the houses of Shem and let Canaan become his servant.”

I. This raises two points, the meaning of “becoming sober” and that of the “younger son.” The former is treated briefly. Sobriety is conceived of mainly as sobriety of soul, which takes the same place in the soul as clear vision in the body, and thus provides it with thoughts which in their turn lead to good actions (1-5).

The word “younger” starts Philo on a discussion of the use made in the Pentateuch of words literally denoting age, to shew moral relations. Ham is “younger” because his unfilial and indecent action proved his spirit of (youthful) rebelliousness (νεωτεροποιια) (6). And so Ishmael is called a “child” when, as a little calculation will shew, he was twenty years old, because as a type of the falsely wise or sophist, he is, compared with the wise Isaac, a mere child (7-9). So too Moses calls the rebellious Israelites “blameworthy children” (10-11). Rachel (bodily beauty) is called younger than Leah (beauty of soul) (12). Joseph’s “youth” in the moral sense is shewn by his staying in Egypt (the body) and his association with his illegitimate brethren (12-15). Conversely the wise Abraham is called the “elder,” though the history represents him as less long-lived than his ancestors (16-18). The elders Moses is directed to choose mean those whose sterling worth he has proved (19-20). In particular the enactment forbidding the disinheritance of the firstborn son of the hated wife in favour of the younger son of the beloved wife, which gave rise to the long allegory of De Sacrificiis, 19-44 is audaciously pressed into service. As in De Sacrificiis the beloved wife is Pleasure, the hated Virtue, but as Moses mentioned the parenthood of Pleasure first, her child is firstborn in point of time and the name only belongs to the child of virtue in consideration of his moral superiority (21-26). So the younger in age Jacob takes the birthright from the elder Esau, and Jacob sets Ephraim who represents the faculty of memory, which comes later and is therefore younger, above Manasseh, who represents the more childish faculty of recollection, which is earlier and therefore older (27-29). This division ends with a statement of the justice of cursing the “younger” (30).

II. But why did Noah curse Ham’s son Canaan, against whom nothing is alleged, instead of Ham? (31-33). Because while Ham is evil potential or “in rest,” Canaan is evil active or “in motion.” To understand this we must consider these terms “rest” and “motion” with their respective congeners, “habit” or “faculty” (εξις) and “activity” (33-34). Now every workman or artist is called by such a name, even when he is not making anything, because he still has the faculty. But it is only when he is actually plying his trade or art that he incurs praise or blame (35-37). So too in the moral sphere. The possessor of good or bad qualities may have no opportunity for displaying them, but the qualities are still there (38-43). Ham means “heat,” i.e. the latent disease in the soul, Canaan means “tossing,” which represents the same in active motion. As no ruler punishes qualities till they actually produce crimes, Canaan properly incurs the curse, though, as one passes into the other, one may say that Ham is cursed through Canaan (44-47). Actual sin is the child of potential sin, and this is the real meaning of “visiting the sins of the fathers upon the children” (48). The same lesson is taught by the law of leprosy that only when the “bright spot” ceases to be stationary does the man become unclean (49), and also by God’s word to Cain, “thou hast sinned, be still” (50).

III. The prayer for Shem speaks of the “Lord, the God of Shem.” Shem is “the good” in its generic not in any of its special forms, and therefore to assert that God is Shem’s God is to put the good man on a level with God’s work, the Universe (51-54). And since “God” indicates the loving side of the Divine Nature, to say that the Lord is “Shem’s God” is to say that, like Abraham, he is God’s friend (55). And here Philo, adapting the well-known Stoic paradox, lays down that such a one alone is noble, rich, king and free (56-57). Finally the word “blessed” applied to God means that he who is thus blest can only repay God by blessing Him (58).

IV. In interpreting the prayer for Japhet Philo passes for a moment into one of his less austere moods. He suggests that the word “widen” means that Japhet may find good not only in the morally beautiful (το καλον) but in the “preferable indifferents” of the Stoics, bodily and external advantages (59-61). As to the last half, “let him dwell in the houses of Shem,” the “him” may be God (Philo ignores the fact that in this case it could not be a prayer for Japhet), for God’s fitting dwelling is in the good man’s soul in the sense that it is especially under His care (62-64). And so in the literal narrative Shem is very properly represented as the ancestor of the Twelve Tribes who are called God’s “palace” (65-66). If “him” is Japhet we may see a correction of the prayer for his “widening,” a prayer that though for a time he may find good elsewhere, his final home may be the excellence of the soul (67-68). The treatise concludes with a few lines on “Canaan shall be their servant.” The fool is indeed the slave of the virtues, if possible, for his reformation and emancipation, if otherwise, for chastisement (69).