One of the most important principles for the Theory of Knowledge is Aristotle's dictum: "Nothing is in the intellect that is not first in the senses". All knowledge begins in sensation, but it does not end in sensation; rather, it ends in intellection (supra sensible knowledge: the knowledge of existence and the natures of things). Our intellectual knowledge, however, shares in the limitations of sense perception. In other words, we come to know what things are gradually, as we perceive the world around us. Moreover, we only perceive things from a limited angle; the more angles from which we perceive a given phenomenon, the more complete is our perception and the intellectual understanding that proceeds from it. Thus, we are always subject to an "availability heuristic".
In philosophy, we distinguish in order to unite. If we fail to make distinctions, our knowledge remains indistinct, that is, confused. What we distinguish in the mind, however, is not necessarily separate outside the mind - in other words, distinction is not separation. For example, we distinguish between substance and accident. The two, however, are not separated outside the mind. We live in a world of "things" or substances (i.e., water, gold, oak trees, birds, oxygen, etc.), and these things have modes of being that are "accidental", that is, these modes of being "inhere in" substance and modify it, not absolutely, but relatively. In other words, a substance can change accidentally (i.e., change color, or size, or position, etc.) while remaining the same substance.
Within the history of philosophy, however, distinction soon became separation - at least for some thinkers - , and all the logical implications of that separation were slowly unravelled. If all knowledge begins in sensation, and all that my senses provide are sense impressions (i.e., color, texture, flavor, odor, temperature, etc.), then what justification do we have for positing the existence of a "substance" underlying these accidental modes of being? Perhaps that's all anything really is, namely a conglomeration of relatively stable sense impressions. Perhaps "substance" is a construct, something we produce, or perhaps it is an a priori category. And if sense impressions are "in us", how do we really know that there is actually anything "outside of us"?
To overcome these difficulties, I will argue that it is best to pay close attention to what it is we perceive when in the act of perceiving. I do not see color, nor do I feel texture, or taste flavor, etc. Rather, I see an apple, or more specifically, I see a red apple, and I taste a sweet apple. In other words, I sense the substance. The reason I say this is because nothing is in the intellect that is not first in the senses. If substance is not in the senses first, then how does it end up in the mind? If it is not first in the senses, then it is something that I construct or impose upon the sense data. One would then have to explain why I do that, or feel the need to do that. And perhaps "I" am nothing more than a conglomeration of sense impressions, that is, perhaps there is no such thing as a single "self" that underlies the sense impressions normally associated with the self. In other words, we really don't know if there is any such "thing" as a "self", or a world outside the self.
All this follows from separating what is in fact only distinct. Substance is in the senses. The idea that it is "sense data" alone that is in the senses is an abstraction. I do not sense "sense data", I sense beings, material beings that are sensible , that is, material beings that are able to be sensed by virtue of sense qualities (accidental modes of being) they possess. The apple is able to be perceived because it is colored, because it is hard, because it is extended, etc. I do not touch hardness; I touch a hard thing, and so on with the other senses. To claim that we perceive our sense impressions is to turn a distinction into a separation; it is to "reify" an abstraction.
Some continue to argue: "No, you believe that there is a substance underneath your perception." But the very act of believing arises because there are things we cannot see, for example, a person's intentions or motives. There are causes that we do not always see, and so we make inferences, and we trust those inferences; but inferences have to be tested. In other words, we require some empirical evidence for the belief so that it may cease to be a belief and become "knowledge". For example, I heard a knock at the door, and I believe it is the kid next door playing games again. That hypothesis has to be tested, and so I study the footage from the outdoor camera. I see it is him after all; I no longer believe it is him, I know it is him. Granted, the process of moving from belief in a hypothesis to knowledge is rarely so simple, and most of what we believe remains belief. My point, however, is that one is not justified in claiming that we believe there is a substance behind the perception, that is, behind the red, the wide surface that is round, the hardness, etc., because belief is intelligible only against the background of perception. Distinguishing substance from accident and then taking substance out of the picture entirely is arbitrary. On what basis does a person claim that "substance" is "a belief"? The answer is: "On the basis that I cannot see the substance". But that is precisely the point; I can see the substance; that is precisely what I see. I don't see red, just as I cannot show you red; I see an apple that is red, and I can show you an apple that is red, that is, a red thing. Thing or substance is in the mind because it is first in sense perception.
Moreover, if we did not perceive substances or beings, then we could not make a distinction between a perception and an illusion - or hallucination. Those two concepts would remain unintelligible except against the background of what is not an illusion or hallucination, namely, an ordinary perception.
There is more to what we see, however, than the simple content of a perception. There is very often an active interpreting that takes place, one that often leads to a misperception, using perception in a much wider sense. After speaking with a professor about student participation and interest, I found that although I disagreed with his claim that students today are indifferent, I was nonetheless unconsciously influenced by his claim, which was an inference, a hypothesis formulated on the basis of some evidence; as such, it required testing. Now it is possible that my rejection of his hypothesis was not based on as much evidence as he had to support his claim, but on fear - I don't want to believe that students are indifferent. Nevertheless, his claim was there in the back of my mind, and one day the students of my class were simply not answering any questions; all I got from them were blank stares. I noticed one student in particular; what I perceived was a student who was utterly bored, unmotivated, not interested, completely indifferent, and that perception caused me to feel angry: "Why is he here if he is not interested in philosophy? Does he expect me to do a song and dance for him? Does he expect me to just deposit information without any contribution or inquiry on his part?" His apathy continued, and I eventually reacted with anger and warned him, only to find out later, at the end of the period, that he was not bored at all, not indifferent, but feeling terribly sick and about to vomit. He was very polite, as he always has been, but he simply did not know what was wrong with him that morning.
I perceived a bored and indifferent student. I misperceived. My perception was "theory-laden", for a hypothesis was there in the back of my mind ready to mould the data provided by my senses. I did not, however, perceive a bored and indifferent student, but a sick student who was coming down with something. My perception was in part a construct, for it was under the influence of an inference at the back of my mind, one that had only a probability of being true, an inference that required testing or further evidence, and one that was not mine to begin with, but someone else's.
My perception was accompanied by an interpretation. Do we ever perceive anything without expounding on it, that is, without explaining it in some way, without interpreting it, unpacking what it possibly means? Do we ever perceive without constructing a narrative? Is there really such a thing as a neutral observation? For the most part at least, I don't think there is. And so although I would argue that absolute skepticism is indefensible, I believe it is very important to cultivate a healthy skepticism in the face of our own knowledge claims. Our perceptions are very often - if not always - mixed with narrative constructions and influenced by unarticulated hypotheses.