I am reading "Ancient Philosophy" by Anthony Kenny (Vol 1 of his "A New History of Western Philosophy", OUP, ISBN 0–19–875273–3).

I was intrigued by the following statement (p.176):

"However, ancient epistemology is bedevilled by two different but related fallacies. Both of them are generated by a misunderstanding of the truth that whatever is knowledge must be true. One of the fallacies haunts classical epistemology, up to the time of Aristotle; the other fallacy haunts Hellenistic and imperial epistemology."

For the sake of limiting the scope of discussion, my question is about the first fallacy. Here's how Kenny describes it:

"The first fallacy is this. ‘Whatever is knowledge must be true’ may be interpreted in two ways.
(1) Necessarily, if p is known, p is true or
(2) If p is known, p is necessarily true.

(1) is true but (2) is false. It is a necessary truth that if I know you are sitting down, then you are sitting down; but if I know you are sitting down it is not a necessary truth that you are sitting down; you may get up at any moment. Plato and Aristotle, over and over again, seem to regard (2) as indistinguishable from (1). Given the necessary connection between knowledge and truth, they seem to think, only what is necessary can be known. From the acceptance of (2) there flows the construction of the theory of eternal and immutable Ideas, and there flows the impossible ideal of Aristotelian science." (p.176-177)

My question is, what exactly is Kenny saying about Plato's and Aristotle's epistemological views?

I mean, it seems that he's talking about contigent vs. necessary truth, and that Plato & Aristotle thought of all truth as necessary -- but I don't understand what that really means in terms of their theories. Is it being claimed that if Aristotle saw a blue unicorn, he'd assert that all unicorns are necessarily blue? I.e. that he would not be able to conceive of (the truth of) the color as being contingent on the particular unicorn? Or is it being claimed that he'd consider the blue-ness of a specific unicorn to be something less than truth, precisely because this blue-ness is not necessary of other unicorns? Or something else?

Kenny closes by making a strong claim about the "impossible ideal of Aristotelean science". I think here he ascribes to Aristotle the theory that science should be a deductive (via syllogisms) process, producing only "necessary" truths. But is calling it an "impossible ideal" really justified, especially in the sense of Aristotle committing some kind of clear fallacy..? After all, didn't the logical positivists take up exactly this line of thinking about truth, and haven't they come up with some interesting/useful/influential arguments about what science is, even if other approaches have become more popular..?

  • 3
    Hintikka J., Knowledge and the Known: Historical Perspectives in Epistemology (1974), the best place to check that I can think of.
    – sand1
    Commented Feb 23, 2018 at 14:57
  • I'm with Plato and Aristotle if you've described their view correctly. If we know something it must be the case for otherwise we could not know it. The problem may be that often we think we know things when we don't, but Cartesian doubt usually sorts this out.
    – user20253
    Commented Feb 24, 2018 at 11:41

1 Answer 1


The basic issue at hand is the location of the necessity or to put it another way -- to what does necessity apply. I don't think it's quite as viscous as your blue unicorn example makes it out to be -- because I think you're jumping from particular to universal in a way that they are not required to do.

To state it a bit more carefully,

I know there is a blue unicorn.

What follows is

  1. Because I know there is a blue unicorn, that unicorn is necessarily blue.


  1. Because the unicorn is blue, there is a necessity that this unicorn be blue.

i.e., my knowing fact Q necessarily entails Q is true. But it does not entail Q is necessarily true. The necessity on the modern picture lies squarely on the transitivity from knowledge; the classical picture does not distinguish things so carefully.

Or to put it another way, you could understand Kenny's claim as a claim about the direction of the "arrow of necessity", known things are necessarily true, true things are not necessarily so.

Added to try to again

It's important to not confuse the issue. Aristotle is not committed to something like this:

  1. I know this dog has yellow fur
  2. Therefore, all dogs have yellow fur.

He is committed to this:

  1. I know this dog has yellow fur.
  2. This dog by necessity has yellow fur.
  3. Yellow fur is a necessary property of this dog.

Step 2 is trivially true assuming that knowledge is understood as knowing the truth of something (the necessity then would come from the definition).

Step 3 however is not at all trivial and that's where things get messy in ancient vs. modern. Aristotle on several occasions (I don't have the cites with me but think this happens in the Posterior Analytic and Organon and Metaphysics and Biology) seems to commit himself to the claim that 3 is also true.

On the modern account we don't commit ourselves to anything about the metaphysics of the object in question -- just it's current state. And that's what Kenny is making a big deal about.

I don't know if I've ever read that particular text by Kenny but two things occur to me. It's not like Kenny has no stake in this particular debate; he's got a position that depends on modifying Aristotle/Aquinas quite a bit and in particular with respect to the understanding with form. So it's not surprising that he thinks there is something wrong with Aristotle's belief (in continuity with Plato) that form is immutable.

Two, it does make sense that the task of science would have trouble proceeding if all truths are assumed to be necessary. Contingency is a key feature in being able to test situations and do experiments. For instance, if we had the following:

  1. given conditions A,B, and C, then Q.
  2. just given A and B, no Q.
  3. with just A, Q.

Then this sort of variation requires us not to take the necessity of our knowledge meaning something is true as projecting into the truth of something necessitating it's truth.

That's my best reconstruction having not read Kenny (in general) in a bit.

  • Thanks, this definitely adds some clarity. I think I obviously need to read more on the subject, because at this point I don't really understand the "big-picture" implications of this distinction; it's also a bit difficult to fathom that Aristotle would not understand, at least in some way, the difference between, let's say, "all Greek bachelors are unmarried" vs. "all Greek bachelors are less than 8 feet tall"...
    – Alex Sotka
    Commented Feb 24, 2018 at 0:29
  • I'm not following on why you think Aristotle can't distinguish those two cases, because I think you're jumping to generalize in a way he doesn't have to. You're right that the basic idea is that for Aristotle knowledge is knowledge of essences (primarily) and only then accidents.
    – virmaior
    Commented Feb 24, 2018 at 0:36
  • I was referring to Kenny's comment that "Plato and Aristotle ... again, seem to regard (2) as indistinguishable from (1)", as being a fallacy that "haunts classical epistemology". It seems that Kenny is exactly saying that Aristotle failed to grasp this difference -- to which my initial reaction is: 1. If we're talking about simple examples, it "seems" that Aristotle had more than enough mental power to understand the difference. 2. If we're talking about something "narrowly technical", then it makes me wonder what Kenny means by going so far as to call it a "haunting" fallacy.
    – Alex Sotka
    Commented Feb 24, 2018 at 1:28
  • I don't think you're understanding Kenny correctly on that nor do I think he's referring to something narrowly technical. He's referring to their confused understanding of the relationship between knowledge and necessity, but it's not confused in the way that you seem to be taking it. They're confused in that they let necessity apply transitively / in both directions when it only applies (on the modern understanding in one direction. I tried to explain this in my answer...
    – virmaior
    Commented Feb 24, 2018 at 1:39
  • I really appreciate the added explanation, esp. that bit about the "metaphysics of the object vs. current state" rings important... Gonna try to chew through this stuff carefully.
    – Alex Sotka
    Commented Feb 24, 2018 at 2:02

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