G. Reale, A History of Ancient Philosophy: The Schools of the Imperial Age, trans. John R. Catan (Albany: State University of New York Press, 1990) 111.
Reale discusses Aenesidemus, whom he dates to the first century B.C.E. Aenesidemus was a significant figure in the rebirth of Pyrrhonism, which is a form of skepticism. Thus, Aenesidemus appears in Reale's section on Neo-Skepticism.
Here is a quote:
...in the same individual not only are the structures of the senses different but so are the dispositions, conditions, states of mind different and changeable, all of which consequently condition the representations. It all verifies the conclusion that our representations differ according to whether we are happy or ill, young or old, in our right mind or out of it, happy or unhappy and thus as a consequence, for this reason as well, we must suspend judgment.
How well can human beings understand and conceptualize the outside world? We look at this world from our own perspectives, and our conclusions about what we see can be different. Some people notice one detail, while others observe another. And then there is the question about whether people are interpreting the detail that they see in a correct manner.
Memory can also be flawed. I once saw an episode of Frasier in which Frasier and Niles remembered an event quite differently. The event was this: Niles said that he was content as he put his arm around Daphne, and Martin said he was too as he put his arm around Ronnie (Martin's girlfriend, aka Nina on Just Shoot Me). And, in Nile's recollection, he and Martin made those remarks in a normal voice. But Frasier remembered them saying it in a snide manner, as if they were bragging about their own success in love as compared to Frasier's failure. Frasier heard "Nyah, nyah, nyah, nyah, nyah" in his recollection. There's one event, but two different recollections.
It often baffles me how I can mis-remember things that I see and hear. When I watch something on TV, I remember what I see in a specific way: this person said this in this manner as he was standing here, looking like this. Then, I watch the same scene months or years later, and what I see is not what I remember. You'd expect the pictures in my head to be accurate, but that's not always the case!
In my understanding, post-modernism says that we can't objectively know the outside world, since we look at it through our own culture, biases, etc. I do not consider myself to be a hard post-modernist, for I agree with something that Jon Levenson said when I was at Harvard: Sure, there is subjectivity, but that doesn't mean that there's nothing but subjectivity. We can know some things with a degree of accuracy, even if our knowledge is not perfect.
At the same time, there is a lot of subjectivity! Academia, for example, has a great deal of group think. One reason that people like the New Perspective on Paul and Judaism (i.e., the view that Judaism was not legalistic) is that they want to avoid Christian supersessionism, which led to the Holocaust. But the scholars before them had the opposite kind of group think: they were influenced by the New Testament to view Judaism as legalistic, a religion that promoted a works-based salvation, in contrast to Paul's emphasis on God's love and grace. One idea is the fad now, whereas another idea was the fad not that long ago.
I've heard scholars of Judaism claim that you can't use the Mishnah to understand the first century Pharisees, since the Mishnah is a later document. In a class I once took, a student tried to understand the Gospel story of the disciples plucking grain on the Sabbath in light of the Mishnah's rule prohibiting harvesting on that day, and the professor criticized him for that. "The Mishnah was later than the New Testament." But I've often seen scholars use later sources to understand earlier ones. Both are from the ancient world, so, with the Mishnah and the New Testament, why can't a person use one source (the Mishnah) to understand another source that was written only slightly earlier (the New Testament)? The answer: academia has its fads that we're supposed to honor (for whatever reason)! We're told that scholarship is all about finding an objective truth, but academia is conditioned by its own culture, perspectives, group think, etc.
But that's just my view, based on my own experiences. Others may argue that reality is more complex than that!