We have been covering this point extensively in the Lucretius Today podcast in recent weeks, but I haven't previously posted the material that Bryan has prepared as part of his Usener material. We've discussed this in our most recent Zoom meetings and I think Bryan's chart helps in focusing on the nature of what is really going on. The text is taken from exactly what we have been covering in Book 2 of Academic Questions:
The manner in which this is stated is unfamiliar to most of us and there are probably some really important things to observe about why it is stated in this form. As i see it one of the real obstacles we face in understanding what the Academic Skeptics were doing is that we today don't use the word "perceived" the way Cicero seems to be using "percipi."
When we say that we perceive something I think we see that as a fairly loose statement on the order of "what i see is..." or "what i hear is...." and we don't equate "perceiving" with "judging that the appearane is true to the facts." We might say "I perceive that that is Plato a hundred yards down the road walking towards us." In saying that, we wouldn't mean "I know beyond a shadow of a doubt that that is absolutely Plato walking toward us, and there is no possibility that it is anyone else."
And so after saying "I perceive that to be Plato walking towards us" we wouldn't be entirely surprised if we turned out to be mistaken, and as the person gets closer we find out that it was someone else.
But when the debate was being conducted in the ancient world, what they apparently meant by "percipio" was something certain, a judgment that could not possibly be false. They were using "I perceive" to mean "I know." So that's at least part of the reason why they framed the debate in terms of "false appearances exist' and 'false appearances cannot be perceived." They are saying something very close to "something I know to be true can be false."
And then in points three and four they are carrying that to its logical conclusion. If something I know to be true can be false, then we have no way of distinguishing between a judgment that is true one that is false. Knowledge is therefore impossible.
Further, it's important for us to know that this is where the Stoics sided with the Skeptics -- both of them against Epicurus.
The Stoics agreed that "some appearances are false." This is why Lucullus said in section 7 of Academic Questions Book Two, that unlike Epicurus, he (as a Stoic) was not going to defend the idea that all sensations are true:
VII.
Let us begin then with the senses — the judgments of which are so clear and certain, that if an option were given to our nature, and if some god were to ask of it whether it is content with its own unimpaired and uncorrupted senses, or whether it desires something better, I do not see what more it could ask for. Nor while speaking on this topic need you wait while I reply to the illustration drawn from a bent oar, or the neck of a dove; for I am not a man to say that everything which seems is exactly of that character of which it seems to be. Epicurus may deal with this idea, and with many others; but in my opinion there is the very greatest truth in the senses, if they are in sound and healthy order, and if everything is removed which could impede or hinder them.
So iulike Epicurus, Lucullus and the Stoics won't defend the idea that ALL sensations / appearances are true. Instead, they argue that although SOME appearances are false, SOME are also true, and there's a way to know the difference. They argue that some impressions are so clear that they cannot possibly be mistaken (this is the kataleptic impression).
As a result the Stoics have to find some way other than through the senses themselves to determine which senses are true and which are false.
And on that point Epicurus agreed with the Skeptics: there IS no such method. If you agree that the sensations and appearances cannot be trusted, then there is in fact no way - no evidence - that will allow you to distinguish true from false.
And this is the great divide:
Epicurus holds that the appearances / sensations CAN be trusted because they are prerational - they are not in fact giving you opinions or judgments in the first place. They can't be false to the facts because they are not in fact giving you an opinion about facts at all. Opinions are formed in the mind, not in the senses. It is your mind that can be wrong because the mind has to use proper reasoning and logic about what it is seeing to determine what is true.
The Skeptics hold that the senses / appearances CANNOT be trusted, but they also don't accept that there is any method outside the senses to determine truth. They cite many arguments for this, including the fact that people disagree on what they are sensing, such as how something tastes or whether it is cold or hot. The Skeptics therefore conclude that it is absolutely impossible to ever be sure of anything.
The Stoics say that the senses cannot be trusted, but if we tune ourselves sufficiently to the divine order of the universe we will realize that some things are so clear that - even though the senses themselves cannot always be trusted - we will know with certainty that some sensations are true. But the Stoics also admit that only the truly wise man can do this, and they admit that the truly wise man is as rare as a phoenix, so they don't generally claim to be truly wise. Cicero correctly points out that this leaves the Stoics in the position of having to admit that virtually all of them don't even know if they are standing in their own house! Cicero's right, because the Stoics say only the truly wise man can know anything, and most of them won't claim to have hit the summit of virtue and wisdom. So the Stoics claim to defend knowledge, but then admit that knowledge is practically impossible to obtain.
Only Epicurus provides a logical defense of the possibility of practical knowledge.
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- Bryan (or others) - if you can improve or correct what i've written there please do!