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 το χειρον τω κρειττονι φιλειν επιτιθεσθαι. Quod deterius potiori insidiari soleat (Mangey, i. 191-225). On Gen. iv. 8-15. The book is already quoted by Origen under this special title (Comm. in Matth. vol. xv, c. 3). Eusebius mistakenly quotes under the same title several passages belonging to De confusione linguarum (Praep. Ev. xi. 15). In the Florilegium of Leontius and Johannes several passages are cited from our book with the formula εκ του ζ και η της νομων ιερων αλληγοριας. Also in Johannes Monachus ineditus (Mangey, i. 191, note). The unusual formula εκ του ζ και η must surely mean, that the seventh book was according to another computation also called the eighth (εκ του ζ του και η would thus be the more accurate). This book then is according to the usual numbering the seventh, but was, in consequence of De opificio mundi being placed first, also called the eighth.” (The Literature of the Jewish People in the Time of Jesus, p. 333)
F. H. Colson and G. H. Whitaker write (Philo, vol. 2, pp. 198-199):
Cain and Abel signify to Philo opposing principles, love of self and love of God (32). The call to Abel to go out into the “plain” is a challenge to a disputation. The opening of the Treatise is mainly occupied in showing that in Genesis “plain” suggests a contest of opposing principles. Why does Jacob call Leah and Rachel to the plain? Because it is there that he “tends his flocks,” i.e. disciplines his lower impulses. The plain is the obvious place for Joseph, the wearer of a many-coloured patchwork of inconsistent tenets, to be sent to by his father that he may be taught better by his brethren, who are there becoming proficients in the work of disciplining their lower nature (their flocks). Even Isaac, going out into the plain to meditate is, in Philo’s eyes, the peerless champion, who finds the field emptied by the retirement of all his adversaries. “The plain,” says Philo in 32, “has now been shown to be a figure of a contest,” and so he passes on to his next point.
Abel was ill-advised to accept Cain’s challenge. Self-love can plead for itself (33 f.) with an eloquence which can be met only by one versed in dialectic, and Abel lacked such training. Moses was wiser in shrinking from meeting the sophists of Egypt, acknowledging himself to be without eloquence, nay, devoid of speech itself, and waiting for “Aaron,” who commonly represents for Philo the uttered word. Thought should ever be wedded to speech. Glib fools are contemptible, but dumb wise men are ineffective (44 ff.). This is a theme to which Philo returns later on (126 ff.), where he enlarges on the joy of speech in interpreting thought.
Yet the seeming victory of the false view is really a defeat (47), as is evident when we consider well what is implied in the words, “the voice of thy brother’s blood.” Here is the great truth, which is plainly stated in Lev. xvii. 11, that “the Life is in the Blood.” The Life which is Life indeed emerges from seeming death no longer “speechless.” It has now a “voice,” which God hears (47 ff. and 92 f.). This theme is taken up again in 70 ff., where the question put to Cain, “What hast thou done?” is treated as equivalent to “Thou hast effected nothing,” and as signifying the futility of sophism, ‘clothing itself with’ Balaam or anyone else, in contrast with the undying life of virtue.
The seeming victor, moreover, brings on himself a curse which comes to him “from the earth,” i.e. the senses which are his chosen field (98 ff.). He may toil at it, but can never till it (104 ff.). It will never second his efforts (112 f.). He must go “groaning and trembling” (119, 129 f.), never finding rest with ‘Noah,’ or laughter with ‘Isaac,’ or joy in himself’ with ‘Aaron,’ or hope with ‘Enos’ (120 ff.). He will taste abandonment (141 ff.) and the shame of exposure to the eyes of God (158 ff.).