Most of the people who reject the Seventh Letter see it as somehow inconsistent with the rest of Plato's philosophy. I am not one of those who reject the Seventh Letter. Plato was constantly experimenting, and so there are going to be differences between each presentation of his main ideas. Moreover, it hardly makes sense to speak of inconsistency in Plato since, unlike Aristotle, almost all of his writings are dialogues. Although Socrates often seems to represent Plato's own point of view, it is by no means clear when he does. Moreover I suspect that many rejections of the philosophical parts of the Seventh Letter are more due to its violating certain intellectualist/rationalist prejudices on the part of philosophers doing the interpreting. I find the Seventh Letter to be consistent with my interpretation of the rest of Plato's writings, at least as far as one can talk about consistency here.
I am using the L.A. Post translation here, found in The Collected Dialogues edited by Edith Hamilton and Huntington Cairns. The passage begins with the striking claim that Plato has not composed any work in regard to his doctrines, and that he won't even do so in the future. (341d) The reason for this is that there is no way to put it into words, unlike other studies: "Acquaintance with it must come rather after a long period of attendance on instruction in the subject itself and from close companionship, when, suddenly, like a blaze kindled by a leaping spark, it is generated in the soul and at once becomes self-sustaining." (341d) Plato began this discussion by talking about acquaintance with doctrines or subjects, but this seems more like acquaintance with a realm of philosophical truth or maybe even a sort of intuitive oneness with the subject matter. We tend to associate philosophical doctrines necessarily with something written. But this is something that happens to the soul, and it is ineffable.
In the next paragraph we learn that this "acquaintance" (hardly the right sort of word for this, it seems) is of "the nature of things." Plato then says, "I do not...think the attempt to tell mankind of these matters a good thing, except in the case of some few who are capable of discovering the truth for themselves with a little guidance." (342e) The term "little" seems misplaced here given that he had just said that there would be a long period of instruction. Interestingly and perhaps with some degree of self-contradiction Plato then goes on to "speak on the subject at greater length" in order to make it clearer. What he is going to give here is "a true doctrine, which I have often stated before, that stands in the way of the man who would dare to write even the least thing on such matters, and which it seems I am now called upon to repeat." So, the point is that the doctrine that follows is preliminary to that which is ineffable and philosophically deep. What follows is not exactly the same as what he said previously, but it is pretty similar, even though he does not use the term "Forms." By "preliminary" I do not mean that there is another set of doctrines that can be explicitly stated. I only mean that Plato believes that this "conversational" method gives you the self-sustaining kindled blaze...which is the whole point.
Plato starts off with three classes of objects "through which knowledge about [the nature of things] must come." He says that "the knowledge itself is a fourth," which is to say that it is a fourth class of things, these in the mind, to consider here. The fifth thing to consider is the "actual object of knowledge which is the true reality."
To go into more detail, the first is the name, the second is the description or definition, and the third is the image. The role of each is interesting to study in detail and this study will reveal some surprises. For one thing it is quite surprising that Plato incorporates the image (eidolon) into his first three classes as something positive. I think this is necessary for Plato. When he attacks the eidolon he only does so when it is mistaken for the real thing. It is always taken as a necessary starting point. In the Symposium one must begin with appreciation of the body of a particular young man, for example. Later we will learn that the role of definition is not quite what we would expect either.
The example Plato uses to explain his theory is a circle. So, in this case, the name is "circle." The definition is "the thing which has everywhere equal distances between its extremities and its center." And the third thing is the class of objects drawn or turned on a lathe. Many would think that the word and the definition would be sufficient for knowledge. But here we have the difference between mere knowledge and wisdom, or at least whatever wisdom is attainable by the philosopher. Wisdom is going beyond definition and knowledge.
It is not surprising that Plato makes a distinction between the true circle and the mere image of a circle in the world of appearances. For example if the drawn circle is erased this does "not affect the real circle to which these other circles are all related, because it is different from them." A little surprising, however, is the further discussion of the fourth, which is now described as three things: knowledge, understanding (nous) and correct opinion, for Plato elsewhere distinguishes between these, and here he seems not to care about that distinction, at least between knowledge and true opinion.
The point he wants to make here is that these epistemic concepts, taken together, are found not in sounds or shapes but in minds, and that the real circle is not found in minds. In any case he sees understanding as the closest "in affinity and likeness" to the fifth entity, the real circle. This, too, might be surprising to some who might give this to knowledge (episteme). But if knowledge is justified true belief then the justification and the belief must be stated in words. Plato, at this time in his life allows understanding (which is not in words) to trump mere knowledge.
He goes on to extend this point to all of the other Forms (or, better, all things that can be said to have essences), for example shapes, surfaces, good, beautiful, just, bodies (artificial and natural), the elements (fire, water, etc.), every animal, qualities, and states. To get a complete understanding of the fifth one must "get hold of the first four." This is striking since one must get hold not only of the name and the definition but also of the image and, presumably, both knowledge and right opinion. The term "get hold of" is not really explained, but seems to mean "gain a firm grasp of these things and their relations."
He then says "Furthermore these four [names, descriptions, bodily forms, concepts] do as much to illustrate the particular quality of any object as they do to illustrate the essential reality because of the inadequacy of the language." The point is that the four illustrate particular qualities, and this may be confused with their philosophical purpose, i.e. to illuminate the essence. This is the reason why he next says that no intelligent man will put into language what his reason has contemplated, i.e. not in written symbols, since writings cannot be changed. (343a) This is similar, as many have observed, to the Phaedrus attack on writing as opposed to knowledge based on personal conversation.
To help explain the meaning of what he has said here Plato notes that circles in the world of appearance, i.e. ones that are drawn in dirt or turned on a lathe, are the opposite of the fifth entity, the real circle. The reason is that they would touch a straight line at several points, and this would mean that they would contain within them their opposite, i.e. straightness. He then observes that names are not stable since you could call what we now call round straight and vice versa. More interesting is that he applies this point to description (i.e. the definition) as well, since the definition is made up of words too, i.e. of "nouns and verbal expressions." So, in general, the four are inaccurate. (Note that he includes knowledge in this group, although perhaps not understanding!}
So there are two things, the essential reality and the particular quality, and "when the mind is in quest of knowledge not of the particular but of the essential, each of the four confronts the mind with the unsought particular, whether in verbal or in bodily form." (343c) So the four by themselves are not sufficient and can actually deceive us, focusing on the particular rather than on the essential. The further problem is that "each of the four makes the reality that is expressed in words or illustrated in objects liable to easy refutation by the evidence of the senses." And this leaves us prey to confusion and uncertainty.
Bad training leads us to accept the phenomenal presentations, including both definitions and knowledge as justified true belief, and not to look for real essences. Those, like the Sophists, who are able to "handle the four with dexterity" can easily make a fool of the individual who tries to provide answers about the fifth entity. The problem is not the mind of the speaker but the character of the four, which is "naturally defective."
So how are we to proceed? There is a method: it is consideration of the four in turn "moving up and down from one another." I take this to be central. All four need to be considered in sequence: it is not enough to move from word to definition, but one must also move then to the image, and then to knowledge. And then one must also work one's way back down again. We must recognize that whatever is in language is changeable.
I think that there is an implicit reference here to Heraclitus' saying that the path up and the path down are one and the same. Plato often talks of two paths, one leading to the Forms and one leading away from the Forms. To say that the path up and the path down are the same is to say, I believe, that wisdom is a process and a cycle in which one mounts to the Forms but also descends from them back to the world of appearances.
As Plato observes, even this procedure "barely begets knowledge of a naturally flawless object in a naturally flawless man." Most people are "defective" in that they have no interest in essences: they are not philosophers.
Plato goes further when he says "most people's minds with regard to intelligence and to what are called morals" are defective. So what is needed to engage in this search for essences is "natural intelligence and a good memory" - but also an "inborn affinity with the subject." That is, one needs to have a passionate attachment to searching for essences, to philosophy, and also, which is the same thing, passionate attachment for self-improvement, achievement of arete.
By natural affinity Plato may mean not simply a philosophical but a moral affinity, for he says "all who have no natural attitude for and affinity with justice and all other other noble ideals, though in the study of other matters they may be both intelligent and retentive" will fail to grasp the entity. Also those who are naturally just and otherwise virtuous may have no intellectual ability and will also fail. So "the study of virtue and vice must be accompanied by an inquiry into what is false and true of existence in general and must be carried on by constant practice throughout a long period..." (344b) This will involve comparing names, definitions and "visual and other sense perceptions." And one must do this in "benevolent disputation by the use of question and answer..." Only then will the "flash of understanding" blaze up, and the mind will be "flooded with light."
All of this it seems to me is consistent both with the Republic discussion of the cave, line and sun, and with the ladder of love as presented by Diotima in the Symposium. When one grasps the Good itself one does not grasp a definition. Rather one is able to see the good in things. When one grasps the Beautiful itself one does not grasp a definition but one is able to see the beautiful in things. Words, definitions, images, and even knowledge itself (justified true belief) are just stepping stones to the flash of insight.
Is this true? Actually I think so, although I couldn't prove it. Also, unlike Plato, I do not think that the real thing, the fifth, is eternal and unchanging. Or at least it is not so except as ideal empty of content. I only am sad that Plato used the circle as his main example I think this confuses things since it makes it appear as though grasping a Form is much like knowing the definition of circle. This, of course, is not his intention.
Note on Secondary Sources. W.K.C. Guthrie provides a useful discussion of this material in A History of Greek Philosophy: V The later Plato and the Academy. Cambridge U. Press, 1978.