The keyword for understanding Ennead II.9, Against the Gnostics is ‘ignorance’ (13.10). Plotinus wants to say that the people who pretend to be the only ones to know the truth are uniquely ignorant about it (9.82-3). Plotinus’ diagnosis of Gnostic ignorance has three parts to it. (1) The Gnostics are proud of their privileged knowledge about the intelligible, so Plotinus begins his attacks by revealing that they are in fact ignorant about the nature of the intelligible world (ch.1-3). In this part of the treatise, Plotinus refutes the Gnostics’ multiplication of intelligible realties and clarifies the structure of the intelligible world, which has only three hypostases. (2) The Gnostics’ censure of the sensible world and its Demiurge manifests their ignorance about the generation, the nature and the maker of this world (ch.4-13). According to the Gnostic myth of Sophia, this sensible world was created as a result of the fall and the passions of Sophia, and it will be destroyed after her repentance. In this context, Sophia in the Gnostic myth is equated by Plotinus with the Soul. The two major errors of the Gnostics consist in their belief in the evil and ultimate destruction of the sensible world, and in their attribution of passions to the creative Soul. Against these views, Plotinus argues (a) that the sensible world is an inevitable result of derivation from the intelligible world, of which it is an imitation, and (b) that one should distinguish the world soul from the individual soul: the former does not suffer passions as the latter does. (3) The libertinism of the Gnostics results from their ignorance about the dependence of the sensible world on the intelligible world. The trace of the intelligible in this visible universe is manifest in the beauty of this world, in general providence (contrasted with the Gnostic belief in particular providence), and in the moral virtues which (in contrast with the magic of the Gnostics) actually purify people (ch.14-18). Because the Gnostics do not know that ascent to the intelligible begins with reverence for its trace in this world, they overlook what is beautiful and virtuous in this universe, building the hope of salvation upon a cheap belief in a ‘particular providence’ which they think will ensure salvation without the need for moral and intellectual effort.
This treatise is of particular interest to Plotinian research. To establish the historical background of this polemic demands the substantial philosophical work of establishing parallels between the Plotinian text and our evidence for Gnosticism. This is especially urgent after the discovery of the Nag Hammadi Codices (NHC) in 1945, a complete English translation of which was published in 2000. Texts of the Gnostics themselves, including the Allogenes and Zostrianus, which are mentioned in Porphyry, Vita Plotini 16, add to what we know of Gnosticism from heresiological literature. Among earlier work on the treatise, the commentary by Dietrich Roloff in 1970, Plotin. Die Großschrift III,8-V,8-V,5-II,9 (Berlin), is rather paraphrastic. Vincenzo Cilento, in Paideia antignostica (Florence, 1971) used the heresiological literature and such few editions of the newly discovered Nag Hammadi material as were available to him. In 1990, the German commentary by Karin Alt, Philosophie gegen Gnosis: Plotins Polemik in seiner Schrift II,9 (Mainz, 1990), established informative parallels between the Plotinian text and the heresiological literature, and also drew on NHC, but sparingly. After 2000, the commentary by Richard Dufour on II.9 in the series Plotin. Traités 30-37 (Paris, 2003), and the commentary by Paul Kalligas, The Enneads of Plotinus. A Commentary (Oxford, 2014), made use of NHC alongside the heresiological literature to good effect; and a recent commentary by Nicola Spanu in 2012, Plotinus, Ennead II 9  ‘Against the Gnostics’ (Leuven, 2012), is also inspiring. Further work on the treatise aims at establishing the identity of Gnostics referred to be Plotinus: this includes Henry-Charles Puech, ‘Plotin et les Gnostiques’ (in Les sources de Plotin, Geneva 960), and Christoph Elsas in Neuplatonische und gnostische Weltablehnung in der Schule Plotins (Berlin; New York, 1975). There has been a lot of discussion of whether Plotinus himself was influenced by Gnosticism, as Hans Jonas suggested. Important is also the work of Denis O’Brien, Théodicée plotinienne, théodicée gnostique (Leiden; New York, 1993), which argues for the position that Plotinus himself endorses the generation of matter from the Soul.
For these reasons, it was ambitious to attempt a thorough new English commentary on II.9. Sebastian Gertz’ publication meets the readers’ expectations. In the tradition of Alt, Dufour and Kalligas, Gertz draws on NHC along with the heresiological literature in his commentary, and provides the readers with an informative reconstruction of the historical background (e.g. pp. 99-102, 125, 133, 254-7, etc.). Meanwhile, he integrates a rich accumulation of research into the commentary, either clarifying particular points or informing the readers of the present state of research. For example, he refers to the works of Elsas (pp. 111, 153), Turner (pp. 102, 161, 233, 248), Narbonne (pp. 138, 148), Rudolph (pp. 215, 240, 262), Schmidt (p. 230), O’Brien (pp. 134, 235-7), Spanu (pp. 246-7), Bazán (pp. 111-2), Dillon (p. 116), Igal (p. 217), Burns (pp. 161, 217), and others.
No important issue is missed in the discussion, including, for example, the controversial thesis of Harder that III.8 , V.8 , V.5  and II.9  originally form a single long treatise (p. 20, n. 10); the observation that II.9 is not directed to the Gnostics themselves, but to those of Plotinus’ disciples who were influenced by Gnosticism (p. 19); the reading of II.9 as ‘culture wars’ (p. 173); the inconsistency between ch. 4 and ch. 10 concerning the myth of Sophia (pp. 212-3), etc.
There is some novelty in the translation. At 1.4-5, καὶ γὰρ αὕτη οὐκ ἄλλο, εἶτα ἕν, οὐδὲ τοῦτο ἄλλο, εἶτα ἀγαθόν, Gertz translates: ‘since this nature [sc. the One] itself is not something or other, and then one, nor is the Good something or other, and then good.’ But it is not evident why ἄλλο is rendered as ‘something or other’: this English expression means irgendetwas, not etwas Anderes. As Gertz notes in the commentary on 1.1-8 (pp. 93-6), the intention of Plotinus here is to deny the self-predication of the One / the Good. Again, Gertz’ reference to Aristotle’s Categories 1a24-5, not given in the other commentaries, is very pertinent in illustrating 1.2-3, οὐδὲν ἔχον ἐν ἑαυτῷ, as denying any attribute (οὐδὲν) to the One (ἐν ἑαυτῷ). But it seems that ἄλλο in 1.4 should also be understood in this sense: ‘one’ or ‘good’ is not an attribute of the One or the Good, but something else (ἄλλο) in which (cf. 1.3 ἐν) the attribute is. So the traditional translation of Armstrong, ‘some other thing’, ‘something else’, would be plainer.
2.4-6, Ψυχῆς δὲ ἡμῶν τὸ μὲν ἀεὶ πρὸς ἐκείνοις, τὸ δὲ πρὸς ταῦτα ἔχειν, τὸ δ' ἐν μέσῳ τούτων is translated: ‘One part of our soul is always directed to the intelligible beings, another part to the things in this world, and another to what is intermediate between those.’ This translation differs from all other translations, which unanimously render τὸ δ' ἐν μέσῳ τούτων as the third part of soul (τὸ δέ) in the middle (ἐν μέσῳ) of the above mentioned two parts of soul (τούτων). It is not rare for Plotinus to use the plural in the place of a dual. Gertz’ interpretation, based on Republic 509d-511e, has not been proposed by the other commentators; but his translation would suggest Greek that read τὸ δὲ πρὸς τὰ ἐν μέσῳ τούτων.
5.11, ἐκ πολλῷ καλλιόνων καὶ καθαρωτέρων: ‘they are much more beautiful and pure.’ This concerns Plotinus’ refutation of Gnostic arrogance: the stars are better than the Gnostics. Other translations are unanimously like that of Armstrong, ‘they are made of much fairer and purer material.’
5.31-2, καὶ εἰ μὲν πρὸ τοῦ κόσμου, ἵνα τί; Ἵνα φυλάξωνται αἱ ψυχαί: ‘and if [he made the paradigm] before our world, for what end [would he have done so]? –So that the souls may be kept safe.’ Gertz, as Dufour, suggests rendering φυλάξωνται as passive (p. 157). This is interesting and perhaps expresses what the text here really means. The problem is whether it is this grammatically possible. The reference to Luke 8:29 does not help to solve this problem, since in Luke 8:29 φυλασσόμενος can be taken as a passive. But are there other examples that can prove that Plotinus or the Gnostics use the subjunctive aorist middle in the place of the subjunctive aorist passive?
7.30-2, Οὔτε οὖν ἐντὸς δεῖ κατέχειν οὔτε ἔξωθεν πιέζουσαν εἰς τὸ εἴσω ὠθεῖν, ἀλλ' ὅπου ἠθέλησεν ἐξ ἀρχῆς αὐτῆς ἡ φύσις μένει: ‘Therefore the soul does not need to hold all beings together from the inside nor push them inside through outward pressure, but from the beginning they remain where the nature of soul wanted.’ It would be more natural to render ὅπου ἠθέλησεν ἐξ ἀρχῆς αὐτῆς ἡ φύσις μένει as ‘they remain where the nature of soul wanted from the beginning’.
Some translations are somewhat free. E.g. in 8.5-8, the words εἰς τὰ τίμια have not been rendered.
In 11.8-11, Εἰ δὲ τῷ λογισμὸν λαβεῖν αὐτὴ (R.J. adopted by Gertz) κόσμου ἠδυνήθη ἐλλάμψαι ἐκ τοῦ λογισμοῦ, διὰ τί οὐχ ἅμα ἐλλάμψασα καὶ κόσμον ἐποίησεν, ἀλλ' ἔμεινε τὴν τῶν εἰδώλων γένεσιν; ‘And if the soul was capable of illuminating this cosmos by grasping, and in accordance with its rational design, why did it not illuminate and create the cosmos at the same time, rather than waiting for the production of images?’ Instead of ‘illuminating this cosmos’, one expects ‘illuminating the darkness’. κόσμου (line 9) is to be construed with λογισμόν (line 8). The direct object of ἐλλάμψαι (line 9) does not appear here, as in line 10. In this case, it means in the myth of Sophia ‘illuminating the darkness, i.e. the matter’.
15.37, ἅπασι πάθεσι is rendered as ‘all kinds of pleasure’. ‘All kinds of passions’ would be more strict, since πάθη include the above-mentioned anger.
Gertz is naturally familiar with the Aristotelian corpus. Some of his references to Aristotle have not been given in other commentaries and are very illuminating to the text (cf. pp. 94, 103, 144, 234).
Finally, some typos, which are not serious: on p. 34, <kai tous theous> should be <kai pros theous>. On the same page, ‘pentapēches’ should be ‘pentapēcheis’. On p. 135, ‘Irenaeus, Against the Heresies, 1.2.4’ should be 1.4.2.