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 ων ιερουργουσιν Αβελ τε και Καιν. De sacrificiis Abelis et Caini (Mangey, i. 163-190). On Gen. iv. 2-4. In the codex Vaticanus the title runs: Περι γενεσεως Αβελ και ων αυτος και ο αδελφος αυτου Καιν ιερουργουσιν. Frequently quoted in Johannes Monachus ineditus with the formula Εκ του περι γενεσεως Αβελ (see Mangey, i. 163, note). Also in the Florilegium of the codex Coislinianus. The missing commentary on Gen. iv. 5-7 would have formed either the conclusion of this book, or a separate book.” (The Literature of the Jewish People in the Time of Jesus, pp. 332-333)
F. H. Colson and G. H. Whitaker write (Philo, vol. 2, pp. 88-93):
The main theme of this treatise is the interpretation of Gen. iv. 2-4.

I. (1-10). He added to this that she brought forth his brother Abel.

II. (11-49). And Abel became a shepherd of sheep, but Cain was a tiller of the land.

III. (50-87). And it came to pass after some days that Cain brought of the fruits of the earth as a sacrifice to the Lord.

IV. (88-end). And Abel brought also himself of the first-born of his sheep and of their fats.

In I. Philo principally meditates on the word “added,” the subject of which he assumes to be God. He holds that addition always implies a removal of something and thus the birth of the Abel attitude of mind, which refers all things to God, implies the removal of the opposite Cain attitude. His thought then passes (5) to the phrase used of the patriarchs “he was added to his people.” He makes comparisons in this respect between Abraham, Isaac, and Jacob, thought of as the three who learn respectively by teaching, nature, and practice, and finally contrasts them (8) with Moses, who is not “added” but translated to God’s presence.

The treatment of II. opens (11) with a discussion as to why Abel the younger is mentioned in v. 2 before his elder brother, the answer being that vice is older in point of time, but virtue in point of worth. This is illustrated (15) from experience of life, for the philosophical calm comes later than the passions of youth, then (17) from the story of Jacob and Esau, and finally (19) by the law of Deut. xxi. 15-17, that the first-born who is the child of the hated wife (i.e. Virtue) is not to be disinherited in favour of the younger child of the beloved wife (i.e. Vice). This leads Philo on to the elaborate allegory (20-44) of the two, as courtesan and chaste woman, pressing their claims upon the mind. Virtue’s harangue, beginning 28, which contains what is probably the most formidable catalogue of bad qualities ever drawn up (32), includes an impassioned eulogy of toil (35-41) and ends with some loosely connected thoughts (43-44) on the inferior value of the secular learning. Her pleading prevails with the mind (45), which becomes what Abel was—a shepherd, and thus we resume the real consideration of the text. The true shepherd controls the unreasoning, but not vicious, faculties (46) and the greatness of the calling is illustrated from various verses in the Pentateuch (48-51). We should here expect some similar interpretation of Cain’s occupation, but Philo dismisses this with the remark that he has treated it in an earlier book (51).
III. The charges brought against Cain in v. 3 are (a) that he offered only “after some days,” (b) that he offered of the fruits, but not of the first-fruits. The first naturally leads to a homily (53) on the duty of ready service. The causes of tardiness are discussed and rebuked by appropriate texts (54-57), and an example of ready thankfulness is found (59) in the story of Abraham, when he bids Sarah hasten to prepare a meal for the angelic visitors of Gen. xviii. Two side thoughts are suggested by this story, (1) an interpretation of the three measures of meal (59), (2) of the phrase “buried cakes” (i.e., cakes haked in the ashes (60) which Philo explains as the duty of reticence about sacred truths; and as this phrase is also used of the dough brought out of Egypt, we are led on to some thoughts about the symbolism of the passover (60-63). We return (64) to the duty of avoiding delay, and Philo dwells on the timelessness of God’s actions, which we should imitate in our worship (64-68). This is contrasted (69) with Pharaoh’s postponement of Moses’ prayers on his behalf, which again is compared with the human tendency to seek help in misfortune from earthly remedies rather than from God (70-71).

The second charge brought against the Cain spirit necessitates an examination of what “first-fruits” are. They must be first in “value,” i.e. virtues (78), but the απαρχη or “first offering” of these is rather an ευχαριστητικος λογος or body of pious meditation. At this point (74-75) Philo, remembering thai in Lev. ii. 14 the offering is to be “new, roasted, sliced, pounded,” passes on to an examination of these four, which are treated with much richness of thought. The substance of our meditation must be fresh inspired thoughts (76-79) which will supersede the old-world learning of the schools, dear as that is to Philo (78). It must be hardened by the fire of close reasoning (80-81). It must be “sliced” or divided by careful analysis and classification of the thoughts under their proper headings (82-85), and finally it must be “pounded,” i.e. made part off ourselves by the discipline of repeated meditation (85-87).

IV. The introduction of the subject of Abel’s offering of the first-born of his sheep is immediately followed by a quotation of the directions with regard to the offering of the first-born in Exod. xiii. 11-13, and the sections 90-117 are almost entirely short homilies on the different parts of this passage. Thus (a) the time of the offering is put at the entrance to Canaan, the “wavering reasoning” from which God means us to escape (90); (b) we have then an apologetic discussion of the words “God sware,” showing that such expressions are a concession to the human tendency to anthropomorphism (91-96); (c) by reading an “if” into the words “and shall give thee,” he draws his favourite moral that we can only give what God has given (97); (d) dwelling on the words “thou shalt set apart” or “separate,” he argues that the ideas of God which we offer to Him must be kept apart from lower and profane conceptions of Him (98-101); (e) “the males to the Lord” means that while the male offspring of the soul are the virtues, those of the “beasts” or senses are such as are kept under control of the mind (102-106); (f) we have an illustrative digression on the similar command in Numb. xv. 19-20, to make offering from the “mixture,” i.e. our compound being, and a contrast with the offerings of perfection, in which there is no setting apart (107-112); (g) on the last verse of Exod. xiii. 11-13, “all that openeth the womb of the ass, thou shalt exchange it for a sheep, but.if thou dost not exchange it, thou shalt redeem it,” we are told that the ass is labour, the sheep progress, and that labour, at least in the case of things indifferent, is futile, unless it brings progress, and if futile must be “redeemed,” i.e. set free (112-116).
At this point the word “redeemed” seems to lead Philo to a different line of thought. What is meant by the saying that the Levites were a ransom or redemption for the first-born ? Levi—’sanctified Reason,’ Israel’s first-born, is accepted by God before Reuben, Jacob’s first-born, ‘natural ability’ (118-121). But it means also that the wise are the ransom for the fools. This was shown in God’s willingness to spare Sodom for the sake of ten righteous, and we see it in the saving influence of good men in a commonwealth, and so in the commonwealth of the individual virtuous thoughts redeem the evil (121-126). This last explains the saying that the cities of the Levites are “ransomed for ever,” for this ransom of the soul is a perpetual process (127). This again leads on to a discussion why these cities were assigned as a refuge for the homicide. The Levite like the homicide is a fugitive—from natural ties (129). He too has slain—wicked doctrines as in Exod. xxxii. (130), and he represents the merciful side of God’s legislative power, as the homicide does the punitive, for he slew “whom the Lord delivered into his hand” (131-133). Finally, when the sanctification of the Levite is assigned to the day when God smote Egypt, we are taught that since that smiting is perpetual, the sanctification is also perpetual (134-135).

We return for a moment to Abel and his offering of the fat, but pass at once to a comment on the fact that neither heart nor brain, the seat of the dominant principle, appear in the sacrificial ritual. Only when this mind of ours has been purged of its tendency to lapses will it be admitted as a proper part of the ολοκαυτωμα or “whole burnt offering” (136-end).

There are two special points in connexion with the text which require mention.

The first is that we have for this treatise and that of Quis Rer. Div. Her. the valuable help of a papyrus discovered in Upper Egypt in 1889. Not only is this papyrus considerably older than the other MSS. to which it is most akin, but the analysis given by Cohn goes far to justify his opinion that it presents on the whole a better text.

The other is the history of the sections 21-32, which do not appear in this place in Mangey’s edition nor in Yonge’s translation. These sections containing the allegory of the two women had been incorporated in an otherwise spurious treatise, De Mercede Meretricis. In consequence the archetype of the MSS. from which Turnebus made his edition of 1552 omitted them here, and this was followed in subsequent editions. That their proper place is in this treatise is shown not only by their presence in other MSS., but also by the evidence that Ambrose, whose treatise on Cain and Abel draws largely from Philo, evidently had these sections before him.