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 των του δοκησισοφου Καιν εγγονων και ως μεταναστης γινεται. De posteritate Caini sibi visi sapientis et quo pacto sedem mutat (Mangey, i. 226-261). On Gen. iv. 16-25. This book was first published by Mangey from the cod. Vat. 381. Much more correctly from the same manuscript by Tischendorf, Philonea, pp. 84-143. Holwerda gave emendations in 1884 (see note 12 above). This book is in like manner as the former quoted with the formula εκ του η και θ της νομων ιερων αλληγοριας in Leontius and Johannes, in the Florilegium of the codex Coislinianus, and in Johannes Monachus ineditus (Mangey, i. 226, note).” (The Literature of the Jewish People in the Time of Jesus, pp. 333-334)
Emil Schürer continues:
“Of these books none is mentioned by its special title in the catalogue of Eusebius, Hist. eccl. ii. 18, while all that follow are quoted under these titles, evidently because Eusebius considers the former to be included and the latter not included in the joint title νομων ιερων αλληγοριαι. To this must be added, that in the Florilegia also, the quotations under the general title extend exactly thus far. It is therefore highly probable, that Philo issued the following books only under the special titles. Nay, it is also evident why this was done, viz. because from this point onwards the uninterrupted text was no longer commented on, but only slected passages. The exegetic method is however quite the same in the following books.” (The Literature of the Jewish People in the Time of Jesus, p. 334)
F. H. Colson and G. H. Whitaker write (Philo, vol. 2, pp. 323-327):
The treatise begins with a denunciation of anthropomorphism and a defence of allegorical interpretation suggested by the statement that “Cain went out from the face of God” (1-7).

What the Lawgiver teaches by these words is that the soul that forfeits with Adam, or forgoes with Cain, the power of seeing God, loses the joy of the quest of Him, experienced by Moses and by Abraham (8-21); and incurs instability, in lieu of the firm standing gained by them through nearness to God (22-32). Moreover, he is ‘wedded’ to the impious view that “man is the measure of all things,” and fails to regard his offspring, as Seth regarded his, as the gift of God (33-48).

The “city builded” by Cain is the creed set up by every impious soul. Its buildings are arguments, its inhabitants the self-conceited, its law lawlessness, its tower of confusion (Babel) the defence of its tenets. Even the lovers of Virtue are forced by the worldly to build such cities for them (49-59).

At this point (§ 60) Philo stops to illustrate, from the instance of Hebron, how names, like ‘Enoch,’ ‘Methuselah,’ ‘Lamech,’ can have two discrepant shades of meaning, as they have when borne by descendants of Cain and when borne by descendants of Seth. He is also led to give examples of that which is later in time being given precedence over what is earlier, as Hebron was placed above Zoan (60-65).

Having now made clear the nature of the creed which the Cain-like soul sets up, Philo turns to its offspring—’Gaidad’ (or ‘Irad’) is the “flock” of untended irrational faculties. ‘Maiel’ (or ‘Mehujael’) means “away from the Love of God”; ‘Methuselah’ is one “incurring soul death”; and Lamech one “low-cringing”; who “takes to himself” as wives Adah and Zillah (66-74).

Here Philo cannot refrain from pointing out the wrongness of a man taking a wife to himself instead of receiving her as a gift from God. He makes an attempt to account for the fact that the self-same expression is used of Abraham, Isaac, and Moses (75-78).

‘Adah’ = ‘Witness,’ and is like success, leading us to think our actions right because of what they bring. Her son is ‘Jobel’ = “one altering,” the remover of Virtue’s boundaries fixed by right reason, making virtues vices (79-93).

Here follow some subtle remarks on Leviticus xxvii. 32 f. (“both shall be holy”), and on the proofs of holiness, and the number 10, all tending to show that the Law is opposed to ‘altering’ (94-97).

Jobel is also the father of rearers of ‘cattle’; and “cattle” are soul-less passions (98 f.).

Jobel’s son is Jubal, the uttered word, “inclining this way and that,” with no sure, firm, speech. He is also the originator of musical instruments, which are inferior to song-birds, but, like articulate speech, capable of such varied utterance, that it is natural that they should be invented by one who knows no abiding, and is son of one who alters all things (100-111).
Adah having been dealt with (79-111), we turn to Zillah, whose name signifies “shadow,” and who is therefore a symbol of the unsubstantial goods of the body and the outside world. Her son, Tubal, bears a name meaning “all in one,” and represents the “health and wealth” which men deem the sum of human bliss. He is, by trade, a ‘hammerer,’ maker of war and munitions of war, for lusts are the real war-makers and batterers of mankind. Verily is he son of ‘Shadow.’ His sister is Noeman or “fatness,” the product of plenty (112-123).

Lamech, his wives and progeny having been dealt with (73-123), we are brought to Seth, in whom the murdered Abel comes to life. His name signifies “Watering,” for the Mind waters the senses, as the Word of God waters the Virtues, which are symbolized by the four “heads” of the river going out of Eden. The word “heads” is used to indicate the sovereignty conferred by Virtues. The “River” is the Word of God, ever flowing for souls that love God.

“Watering” is so apt a figure of teaching, that Philo is soon showing us Hagar, who represents preliminary education, filling her water-skin from the well of knowledge, to give drink to the boy, who is the soul in its first craving for instruction, that he may grow up to be an ‘archer,’ directing arguments with sure aim. But Philo hastens to give us the picture of Rebecca supplying the water of perfection to the servant of Abraham. Her going down to the well of God’s wisdom shows us that a sense of our own weakness is the beginning of stepping upwards. Her pitcher represents the directness of spiritual teaching, in contrast with the earlier, indirect, instruction through the senses and sensible objects, represented by Hagar’s bulky water-skin.

Every detail of Rebecca’s behaviour to Eliezer brings out a characteristic of the true teacher. She addresses him respectfully. She forgets self in her concern for his need. She says “Drink,” not “I will give thee to drink.” She lets the pitcher down on her left fore-arm and tilts it, suiting her action to the ‘pupil’s’ capacity. She does not forget to water the camels, i.e. to encourage memory, for these animals chew the cud; and they are watered from the well, itself a symbol of memory, from whose depths we draw by the aid of a reminder. The readiness of the camel for toil brings Philo to the Water of Marah, and to the tree by means of which the Israelites, after their toilsome march from Egypt, tasted the sweetness that is essential to fruitful toil. Philo cannot pass over the water which the worshippers of the golden calf were made to drink. His main point is that the grinding down of the calf, the symbol, like Egypt and the animals it worshipped, of the body, shows the inferiority of bodily advantages. Then the ear-rings of which the calf was made show the inferiority of hearing to sight, and the greatness of intuition, implied in the words “See that I AM,” words which are equivalent to “Behold My subsistence,” the essence or quality of God being invisible (138-169).
Returning to Gen. iv. 25 Philo deduces from the word “raised up out of” (the earth) the doctrine that God sows nothing futile in our souls. He takes the word “another” (seed) to mean ‘other than Cain’ in one way, ‘other than Abel’ in another way, and goes on to work out Seth’s ‘otherness’ from Abel. Whereas Abel has relinquished all that is mortal, and gone hence to a higher life, Seth, sprung from human excellence, will never relinquish the human race, but be ‘enlarged’ in it. He is ‘enlarged’ in righteous Noah, the tenth from Adam; in faithful Abraham, another tenth; in Moses, wise in all things, seventh from Abraham. The limit of knowledge attained by Seth is Noah’s starting-point; Noah’s limit is Abraham’s starting-point; and Abraham’s limit the starting-point of Moses (170-174).

In the passage with which the treatise closes we have one of the writer’s contrasts. “God hath raised up to me” is contrasted with the folly and impiety of Lot and his daughters, ‘Counsel’ and ‘Consent,’ and with Rachel’s faulty cry to Jacob, “Give me children.” As she learned from Jacob’s rebuke, “Am I in the place of God?” to say “Let God add to me another son,” so let us, if we so err, repent. The gross sin of Onan is rebuked, and the act of Phinehas the “Mouth-muzzle,” is interpreted as meaning that “he put a stop to the revolt within himself, and turned clean away from his own pleasure.” The last words are a reflection, as appropriate to the twentieth as to the first century, that the soul is the theatre of the most dire wars, and that all wars come from disordered souls (175-185).