We learn about the world through our five senses: by seeing, hearing, smelling, tasting and feeling. Sense perception is a primary means by which we acquire knowledge of contingent matters of fact. We can also acquire such knowledge by, for instance, conscious reasoning and through the written and spoken testimony of others; but knowledge so acquired is derivative, in that it must be based, ultimately, on knowledge arrived at in more primary ways, such as by sense perception.
We can perceive something without acquiring any knowledge about it; for knowledge requires belief, and we can perceive something without having any beliefs about it. Viewing any but the most simple visual scenes we see many things we form no beliefs about. However, when we perceive something, we are acquainted with it by its sensorially appearing (looking, sounding, smelling and so on) some way to us. For we see something if and only if it looks some way to us, hear something if and only if it sounds some way to us, and so on. When, based on how they appear, we form true beliefs about things we perceive, the beliefs sometimes count as knowledge.
Often the way something appears is the way it is. The red, round tomato looks red and round; the sour milk tastes sour. But the senses are fallible. Sometimes the way something appears is different from the way it is. Appearances can fail to match reality, as happens to various extents in cases of illusion. There are, for instance, optical illusions (straight sticks look bent at the water line) and psychological ones (despite being exactly the same length, the Müller-Lyer arrows drawings look different in length). In such cases, looks are misleading.
The ever-present logical possibility of illusion makes beliefs acquired by perception fallible: there is no absolute guarantee that they are true. But that does not prevent them from sometimes counting as knowledge – albeit fallible knowledge. Recognitional abilities enable us to obtain knowledge about things from how they perceptually appear. Sense perception thus acquaints us with things in a way that contributes to positioning us to acquire knowledge about them. The central epistemic issues about sense perception concern its role in so positioning us.
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