Knowledge is of what is.
Knowledge is infallible.
Plato is looking for the feature of what is that accounts for the fact that knowledge cant be mistaken. The infallibility of knowledge is a feature (on the epistemological side) that must be matched (accounted for?) by some feature on the metaphysical side.
The cognitive unreliability of the objects of belief.
That is, our judgments are unreliable because and in so far as the things our judgments are about let us down. In what ways do they let us down?
All of these possibilities may come into play (each involves a different sense of is - a different sense in which the objects of belief or opinion can fail to be.
If understanding and true opinion are distinct, then these by themselves things definitely exist - these Forms, the objects not of our sense perception but of our understanding only. But if - as some people think - true opinion does not differ in any way from understanding, then all the things we perceive through our bodily senses must be assumed to be the most stable things there are. But we do have to speak of understanding and true opinion as distinct, of course, because we can come to have one without the other, and the one is not like the other . . . Since these things are so we must agree that (i) that which keeps its own form unchangingly, which has not been brought into being and is not destroyed, which neither receives into itself anything else from anywhere else, nor itself enters into anything else anywhere, is one thing. It is invisible - it cannot be perceived by the senses at all, and it is the role of understanding to study it. (ii) The second thing is that which shares the others name and resembles it. This thing can be perceived by the senses, and it has been begotten. It is constantly borne along, now coming into being in a certain place and perishing out of it. It is apprehended by opinion, which involves sense perception. . . .
(K) Knowledge is of what is. (477a1)
(I) Ignorance is of what is not. (477a3)
(B) Belief is of what is and is not. (477a-b)
It is most plausible to construe these as conditionals:
In (Ke) knowledge = acquaintance: If you know (i.e., are acquainted with) something, then that thing exists.
In (Kv), we have propositional knowledge: If you know something, then that thing is true.
(Kp) seems to dissolve into the other two, depending on whether we take knowledge to be acquaintance or propositional knowledge:
So we can restrict our attention to (Ke) and (Kv).
Necessarily, (Ke): What you know must exist.
Necessarily, (Kv): What you know must be true.
All of these seem plausible enough; but as we shall see, Plato slides from these innocuous sounding premises to rather startling conclusions.
Are the premises innocuous? That is, can they be accepted by one not antecedently committed to the Theory of Forms? (Remember, Plato is arguing for the existence of Forms from features of the concept of knowledge.) To claim that knowledge is infallible seems innocent enough, for all it seems to say is that knowledge entails truth: Necessarily, if you know that q, then q is true.
But Plato slides from this innocuous reading of the premise to a more controversial one: that the things that we know are necessary truths; that what we know is not merely an existent, but something which must exist (a necessary being).
In the case of ise, the transition is from What is known must exist to What is known is a necessary existent.
In the case of isv, the transition is from What is known must be true to What is known is a necessary truth.
But this is a now-familiar modal fallacy, conflating the necessity of a conditional statement (necessitas consequentiae) with the necessity of the consequent of that statement (necessitas consequentis).
Necessitas consequentiae Necessitas consequentis necessarily (if p, then q) vs. if p then necessarily q (p → q) vs. p → q
(p → q) may be true even though both p and q are contingent truths. Hence, it does not entail p → q. Example: Necessarily, if Toms shirt is crimson, then Toms shirt is red. (Being crimson entails being red.) But although Toms shirt is crimson, it is not a necessary truth that Toms shirt is red (since it is not a necessary truth that Toms shirt is crimson). The color of Toms shirt is a contingent matter.
Cf. Parmenides treatment of the claim what exists must exist.
This fallacy vitiates phase one of Platos argument: the argument that takes us from the truism that knowledge entails truth to the controversial thesis that what is known is a necessary truth.
The transition from Knowledge is of necessary truths to The objects one has knowledge about are invariable, fixed, permanent, unchanging - i.e., the Forms.
This appears to be a different sort of fallacy: that of transferring a property of a proposition to the thing(s) the proposition is about. Its not in general true that if p is about x and p has property F, then x has F. (E.g., one may have a complex proposition about a simple object, a short proposition about a tall object, etc.)
Two comments on Platos move in Phase Two:
The slide from knowledgep to knowledgea is partially concealed by the fact that, in both cases, what is known can be described as a being, something that is. Cf. the plausibility of both of principles (Kv) and (Ke):
What you knowp must be (= be true).
What you knowa must be (= exist).
Platos move from invariable truths to invariable objects of knowledge may be made to seem more plausible if one thinks of the bearers of truth-value - the sorts of things that can be true or false - in a non-standard way.
The standard way (nowadays): the bearer of truth-value (and hence the object of propositional knowledge) is what is expressed by a fully explicit declarative sentence, viz., a proposition. Such things are either true (eternally) or false (eternally), and dont go around changing truth-value. So the proposition that it rained in Seattle on March 14, 1876 is, if true, true forever. It wont change in truth-value. It does not differ at all in that respect from the proposition that 2 + 2 = 4. Thus, on this model it is hard to see why necessity should have anything to do with fixity of truth-value.
But suppose we think of the bearers of truth-value (i.e., the things our cognitive states are about) as corresponding not to fully explicit declarative sentences, with all the local and temporal parameters filled in (like It rained in Seattle on March 14, 1876) but as corresponding to what Quine calls occasion sentences, i.e., sentences like Its raining. Such sentences have implicit indexical elements (here, now, I, you, this, etc.) Such sentences are true on some occasions of utterance, and false on others.
So if, on Monday, you have a belief that you express by saying Its raining, and, on Wednesday, I have a belief that I express by saying Its raining, you and I are in the same belief-state. The content of your belief looks (from the inside) exactly like mine. But we believe different propositions: what you believe is true, and what I believe is false. So if all one has to rely on is the contents of ones mind, ones belief-state, one cannot be guaranteed to arrive at the truth. Here, then, is a cognitive state (belief) that can sometimes go wrong.
But now contrast these (seemingly present tense) sentences:
These cannot change in truth-value. And why is that? Contrast them with Its raining. That can change in truth-value because the weather can change. But these mathematical statements have fixed truth-values because they are about objects that dont change. The eternal truth of 3 + 2 = 5 is guaranteed by the fact that the entities involved, and the relations asserted to obtain among them, are not capable of changing in the respects needed to make the statement turn out to be false.
Platos argument in Phase Two now seems much more plausible than it did before. But there is still room to lodge a complaint:
Are there such relationships? Consider these propositions:
These propositions seem to be invariably true, even though they are not about invariable objects. What makes Zebras have stripes invariably true is not the existence of an invariable zebra, but the fact that an invariable relationship exists among ordinary, variable, corruptible, flesh-and-blood zebras.
The discovery, examination, and explanation of such regularities in nature is the business of natural science, for which Plato makes no provision. His idea that things that can move and change are cognitively unreliable, and cannot be known, has the consequence that natural science is impossible!
For natural science - as Aristotle was quick to notice - must take for granted that the things that exist by nature are, either all or some of them, in motion (i.e., subject to change) (Phys. 185a12-13). And physical science maintains that there can be invariable, necessary truths about changeable, corruptible objects.
Instead, Plato supposes that necessary truths are about Forms. If it really is invariably true that zebras have stripes, this is because of some invariable feature of the Zebra Itself, an incorruptible and eternal object of contemplation.
Note that a consequence of the line Plato takes is that propositions that appear to be about sensible, spatio-temporal particulars turn out, if they are to be objects of knowledge, not to be about those things at all. Which is to say, our knowledge gets cut off from the world of experience.
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