Rough Summary:

§1. The Problem

What’s Gerson talking about?

Gerson is talking about the dominant core position of in the ancient Western world through all of antiquity which might be rightly called Platonism.

What’s he not talking about?

He is not talking about the term ‘Platonism’ as used by modern philosophers to stand for some theory under discussion that no one may have ever held. He gives as examples the use of ‘Platonism’ by Quine and Rorty to denote a foil to their own position. I’m sure he also means to exclude the myriad uses of the term in cases such as ‘mathematical Platonism’ or any number of other cases where “abstracts” are given some kind of strong independence.

How shall we come to understand Platonism?

He doesn’t think it’s sufficient to approach Platonism by just characterizing the thought of people who called themselves “platonists”. Gerson also seems uninterested in mere doxography. He means to show commonly agreed upon principles which will allow the understanding of Platonism as a whole.

He goes on to talk about the place of Plato’s writings themselves and their interpretation in the larger context of Platonism. These details are quite interesting for someone familiar with the basics of contemporary scholarship in ancient philosophy, but the aspect most interesting for us is probably the denial that Aristotle was in conflict with Platonism. In fact Platonists regularly helped themselves to so-called Aristotelian ideas when interpreting Plato. Gerson discusses this as follows:

“The use by Platonists of the Aristotelian material is complicated by the fact that it was generally assumed by them that Aristotle was not an anti-Platonist. More precisely, it was thought that the philosophy of Aristotle was in 'harmony' (σύμφωνος) with the philosophy of Plato. As Simplicius put it, Aristotle was authoritative for the sensible world and Plato for the intelligible world. The differences between them are only apparent and stem from the fact that Plato examines the sensible world on the basis of principles drawn from the intelligible world and Aristotle proceeds in the opposite manner.”

§2. The Fundamental Features of Platonism

In this section Gerson offers a wonderful positive sketch of the metaphysical approach of Platonism.
Before giving his sketch, Gerson notes that all varieties of Platonism are committed to what he characterizes as a “top-down” metaphysical approach of all philosophical problems. Platonism is “resolutely and irreducibly ‘top-down’ rather than ‘bottom-up’”, where a “bottom-up” approach is one that “accepts the claim that the most important and puzzling phenomena we encounter in this world can be explained by seeking the simplest elements out of which these are composed.

By contrast a top-down approach “appeals to irreducible, intelligible principles to account for phenomena” such as “human personhood, and the personal attribute of freedom, cognition, the presence of evil, and the very existence of a universe” holding that “answers to questions about these phenomena are never going to be satisfactorily given in terms of, say, elementary physical particles from which things ‘evolve’ or upon which phenomena ‘supervene’.”

Bottom-up positions are most prototypically different versions of materialism, which Gerson defines as “basically, the position that holds that the only things that exist in the world are bodies and their attributes, however the latter be construed. 

He adds that “All materialists, share the view that, even if attributes are taken to be immaterial in the anodyne sense that they are real and that they are not themselves bodies, they are dependent upon bodies for their existence and explicable entirely in materialistic terms. Thus, for the materialist there are no immaterial or incorporeal entities. Hence, the explanation or account of problematic features of life are obviously not going to be 'top-down'. The explanations must begin and end ultimately with bodies or their parts and the scientific laws governing these.”

The Positive Sketch of Platonism:

Because of its clarity and importance to any future work on Platonism we might do I’m simply going to give Gerson’s Positive Sketch of Platonism unabridged:

(1) The universe has a systematic unity. The practice of systematizing Platonism may be compared with the formulation of a theology based upon Scriptures as well as other canonical evidentiary sources. The hypothesis that a true systematic philosophy is possible at all rests upon an assumption of cosmic unity. This is Platonism's most profound legacy from the Pre-Socratics philosophers. These philosophers held that the world is a unity in the sense that its constituents and the laws according to which it operates are really and intelligibly interrelated. Because the world is a unity, a systematic understanding of it is possible. Thus, particular doctrines in metaphysics, epistemology, ethics, and so on are ultimately relatable within the system. More than this, they are inseparable because the principles that enable us to formulate doctrine in one area are identical with those that enable us to formulate doctrine in another. Many scholars have pointed out the unsystematic nature of Platonism understood as consisting of the raw data of the dialogues. This fact is not necessarily inconsistent with the amenability of claims made in the dialogues to systematization.

(2) The systematic unity is an explanatory hierarchy. The Platonic view of the world – the key to the system – is that the universe is to be seen in hierarchical manner. It is to be understood uncompromisingly from the ‘top-down’. The hierarchy is ordered basically according to two criteria. First, the simple precedes the complex and second the intelligible precedes the sensible. The precedence in both cases is not temporal, but ontological and conceptual. That is, understanding the complex and the sensible depends on understanding the simple and the intelligible because the latter are explanatory of the former. The ultimate explanatory principle in the universe, therefore, must be unqualifiedly simple. For this reason, Platonism is in a sense reductivist, though not in the way that a 'bottom-up' philosophy is. It is conceptually reductivist, not materially reductivist. The simplicity of the first principle is contrasted with the simplicity of elements out of which things are composed according to a 'bottom-up' approach. Whether or to what extent the unqualifiedly simple can also be intelligible or in some sense transcends intelligibility is a deep question within Platonism.

(3) The divine constitutes an irreducible explanatory category. An essential part of the systematic hierarchy is a god adduced first and foremost to explain the order of the sensible world or the world of becoming. Platonism converges on the notion that the divine has complete explanatory ‘reach’. That is, there is nothing that it cannot explain. Thus, ontology and theology are inseparable. The Platonic notion of divinity includes an irremovable personal element, though this is frequently highly attenuated. This attenuation in part follows along the diverse efforts to employ both the intelligible and the simple, as well as the divine, to explain everything else. The residual personhood of the divine agent of transient order is retained in part owing to the fundamental Platonic exhortation to person to ‘become like god’ (see #5 below). Additionally, benevolence and providence are viewed as essential features of the divine, equally in an attenuated sense corresponding to the 'depersonalization' of the divine.

(4) The psychological constitutes an irreducible explanatory category. For Platonism, the universe is itself alive and filled with living things. Soul is the principle of life. Life is not viewed as epiphenomenal or supervenient on what is non-living. On the contrary, soul has a unique explanatory role in the systematic hierarchy. Though soul is fundamentally an explanatory principle, individual souls are fitted into the overall hierarchy in a subordinate manner. One of the central issues facing the Platonists was the relation between intellect, intellection, and the intelligibles, on the one hand, and soul on the other. Just as the psychical was thought to be irreducible to the material, so the intelligible was thought to be irreducible to the psychical. All striving by anything capable of striving is to be understood as in a way the reverse of the derivation of the complex from the simple, the sensible from the intelligible. Thus, the intellectual was not an aspect of or derived from the psychic, but prior to that.

(5) Persons belong to the systematic hierarchy and personal happiness consists in achieving a lost position within the hierarchy. All Platonists accepted the view that in some sense the person was the soul and the soul was immortal. Since perhaps the most important feature of the divine was immortality, the goal or τέλος of embodied personal existence was viewed as ‘becoming like god’. But obviously one does not have to strive to become what one already is. The task of ‘becoming like god’ is typically situated within the fundamental polarity in the general Greek concept of nature or φύσις between ‘what is’ and ‘what ought to be’. Thus, normativity is woven into the account of what is objectively real. We are exhorted to become what we really or truly or ideally are. One might say that the first principle of Platonic ethics is that one must ‘become like god’.

(6) The epistemological order is included within the metaphysical order. Modes of cognition are hierarchically gradable according to the hierarchical levels of objective reality. The highest mode of cognition corresponds to the first explanatory principles. All modes of cognition including sense-perception and requiring sense-perception as a condition for their operation are inferior to the highest mode. That persons can be the subject of both the highest mode of cognition and of the lower modes indicates an ambiguity or conflict in personhood between the desires of the embodied human being and those of the ideal disembodied cognitive agent. The conflict is reflected, for example in the differing attractions of the contemplative and the practical.

He goes on to note that it is “misleading to characterize Platonism in terms of dualism(s) like mind (soul)/body or even intelligible/sensible. The hierarchical explanatory framework of top-downism is conceptually prior to these dualisms.” For “Platonism holds that phenomena in the sensible world can only be explained ultimately by intelligible principles. But these phenomena are themselves not coherently characterizable as non-intelligible; otherwise, there would be nothing to explain. So, the putative dualism of sensible/intelligible disguises rather than reveals the fundamental assumption. Again, the dualism mind (soul)/body is secondary to the Platonic position that embodied human existence has to be understood or explained in terms of intelligible ideals. Thus, embodied persons are images of disembodied ideals. If anything, one insisting on dualism as a property of Platonism would be more accurate to describe this as a dualism of embodied person/disembodied person rather than a dualism of mind (soul)/body.”
He notes, interestingly, that Forms are not ultimate principals in Platonism owing to their plurality and internal complexity. 

“Platonism is firmly committed to the existence of an intelligible, that is, immaterial or incorporeal realm, that is ontologically prior to the sensible realm. Thus, Platonism is a form of explanatory realism, in principle similar to theories that posit neutrinos or the unconscious to explain certain phenomena.”
Gerson does not include political philosophy, despite Plato’s famous excursions in the Republic and the Laws. “For Platonists, political philosophy was understood to belong to the discussion of 'popular and political virtue' as described by Plato. This was inferior, albeit instrumental, to the virtue that constituted assimilation to the divine. Consequently, the teaching of political philosophy was basically ignored.”
He explains further, “Platonism maintains a non-materialistic and hierarchical explanatory framework. Specific problems relating to the natural world in general, that is, problems about living and non-living physical entities, cognition, language, and morality, are all addressed within this framework. For Platonism, the sensible properties of things or sensibles themselves are never the starting-points for explanations. The sensible world is always understood as explicable by the intelligible world, that is, by that which is ultimately transparent to an intellect. Specifically, it is an image produced by the intelligible world, though versions of Platonism differ on how to characterize these images. There is nothing self-explanatory about an image. Its ‘real’ inner workings are to be sought in that of which it is an image. Because there is an all-encompassing hierarchy ordered in terms of complexity and intelligibility, the orientation of investigation is thoroughly ‘vertical’ and almost never ‘horizontal’. Thus, there is little room for political philosophy. For political philosophy must start with irreducible political, that is, practical principles. But there cannot be such in Platonism. All principles for Platonism are to be located among that which is relatively simple and intelligible. The concrete and contingent nature of the political militates against the top-down approach.”

“The systematic unity of Platonism can be seen most clearly in its treatment of all matters of cognition. For Platonism, cognition is to be understood, again, hierarchically, with the highest form of cognition, νόησις or ‘intellection’ as the paradigm for all inferior forms, including those which involve the sensible world. The representationalist aspect of all the images of this paradigm is a central focus of Platonic interest. In addition, cognition is what most closely identifies souls or persons, with possession of the highest form of cognition constituting the ideal state. Since the highest form of cognition is a non-representational state, one in which the immaterial cognizer is in a sense identified with the objects of cognition, psychology and epistemology are inseparable from the ontological and theological principles. In short, to understand fully a matter relating to language or belief or rational desire is ultimately to relate those embodied phenomena to the simple and intelligible first principles.”

§3. Platonism by Negation

In this section Gerson simultaneously tries to show both the way you get from the dialogues of Plato to Platonism and to show what might motivate the Platonic position by sketching the philosophical consequences of the arguments made in the dialogues against various non-Platonic positions. I’m not very interested in this section because Gerson does a much better job on detailing this approach in his book “From Plato to Platonism” and because the historical question of getting from Plato to Platonism is only of limited value to us.

That said at this point in his work, Gerson says that negatively defined Platonism is the rejection of nominalism and materialism, and the elaboration of what follows. Gerson’s summary of the dialogues on these points resists easy summation, and should be pursued directly for those interested.

§4. Was Aristotle a Platonist?

This section is largely concerned with historical scholarship, and this amounts to Gerson rejecting the idea that Aristotle was some kind of anti-Platonist, as we noted before.

A core highlight:

"Aristotle is obviously a relentless critic of some theories of Forms. So, evidently was Plato, as we saw above. But Aristotle does not, it seems, deny the ontological priority of the intelligible world to the sensible world. Exactly what sort of priority is this? In many passages in Metaphysics, Aristotle argued both for the priority in substance of actuality to potency and even for the priority in substance of the eternal to the transitory.67 The priority in substance of the eternal to the transitory looks very much like the sort of priority that Aristotle says Plato was interested in.68 Plato held that if X can exist without Y, but Y cannot exist without X, then X is prior to Y in nature and in substance. This is a perfectly reasonable way to understand the Platonic notion of the priority of the intelligible world in relation to the sensible world.

Granted such priority, it will be objected that for Aristotle this intelligible world is a barren terrain, consisting of nothing but the self-absorbed thinking of the prime unmoved mover. An enormous scholarly literature exists on the question of what the this mover is actually thinking of, with the opinion fairly divided. Charles Kahn provides a concise summary of the basis for the interpretation that the prime unmoved mover is thinking of all that is intelligible. He lists four points against what he terms ‘the prevailing view’, namely, that when God knows himself he knows nothing else: (1) At L7, 1072b25 Aristotle says that ‘God has always what we have sometimes’ which picks up b14-15, ‘[God’s] way of life is the best, a way of life that we enjoy for a little time’. If what we sometimes enjoy is contemplation of intelligibles, then God’s superior life can hardly be less than cognition of these intelligibles; it must be cognition of all that is intelligible; (2) At L7, 1072b19-21, it is said that ‘intellect thinks itself according to participation in the intelligible’. This is a strong indication of the meaning of the famous phrase in L9, 1074b33-5 that God is ‘thinking thinking of thinking’. It is by thinking of all that is intelligible that God thinks himself, just as we think ourselves when we think what is intelligible. The difference between God and us is that (a) we are more than the activity of thinking because we are not pure actualities; (b) our thinking is intermittent; and (c) we do not think all that is intelligible at once. But none of these differences contradict the point that God’s perfect self-reflexive cognition includes content; (3) Hence, as suggested by L7, 1072b22, intellect is determined by the essences which are its objects; (4) The claim that if God knew anything other than himself he would be less perfect is spurious, because thinking is identical with its object. As Kahn puts it, ‘the Prime Mover is simply the formal-noetic structure of the cosmos as conscious of itself’ (Kahn’s emphasis). One could dispute all of these points. I only wish to stress that insofar as Aristotle is interpreted as holding that divine thinking has content, he must be seen to be relying on the Platonic principle of hierarchy and of the ontological priority of the intelligible to the sensible."

Also:

“In regard to the immortality of the soul, what Platonism holds essentially is that personal immortality must be understood in an attenuated sense that excludes the idiosyncratic and focuses on the intellect. This is owing to the fact that the contents of intellection are entirely universal. But intellect, when embodied, is evidently a part of the soul and cannot arise apart from soul. These two points are exactly what Aristotle maintains in De Anima: (1) the intellect alone is immortal and (2) embodied intellection in inseparable from specifically psychical activity, that is, activities of the actual composite substance, such as imagination. Again, my point here is simply that with a clear grasp of what Platonism is we should be less willing to suppose that Aristotle is to be interpreted as an anti-Platonist. Accordingly, notoriously difficult passages such as De Anima book three, chapter five may turn out to be somewhat more yielding to our understanding.”

This is great! There's real value and interest in trying to distill what are the deepest metaphysical disagreements between naturalism and religious worldviews, so that they can be clearly and succinctly stated. If it weren't already taken as a title, I'd love to see a book called "Where the conflict really lies" that does just that, show where naturalism and religious worldviews really conflict.