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Samira: Tell me what objects there are in front of you.
Rafa: There’s you in front of me, of course, there’s a table, there’s a book on the table, there’s a bookshelf over there, …
S: Okay, how do you know there’s a bookshelf in front of you?
R: Huh? What do you mean?
S: You said there’s a bookshelf in front of you. How do you know that there’s a bookshelf in front of you?
R: I can see it. There, right in front of me. You’re weird, Mom, you know that?
S: Yes, I know I’m weird. But I’m making a point, and you’ll soon see what it is. You said you know there is a bookshelf in front of you because you see a bookshelf in front of you.
R: How smart!
S: No wise cracks, junior! You see a bookshelf in front of you, and on the basis of what you see, you conclude that there is a bookshelf in front of you.
R: Are we going to do this hair splitting all day?
S: Just one more question. Is it possible that your conclusion based on what you see could be mistaken?
R: Er… I guess it’s possible, like sometimes I see something in front of me, but then I realize that it’s a reflection in the mirror.
S: Right. It could be an illusion, so you might be mistaken.
R: But that bookshelf is not an illusion.
S: I can grant you that. All I’m saying is, our eyes can deceive us, so we shouldn’t trust everything our eyes tell us. A reflection in a mirror is only one kind of illusion. If you put a straight rod in water, you see it as bent. You know it’s not bent but you can’t help seeing that it’s bent. That’s also an illusion. Can you think of other optical illusions?
R: Like the sky being blue?
S: Excellent. You’re catching on. Let’s take another one. Which is bigger, the moon or the stars?
R: The stars.
S: But do you see the stars as bigger?
R: No, what I see is that the moon is bigger than the stars. Oh boy! If I had been born a few thousand years ago, I would have been totally certain that the moon is bigger than the stars.
S: Very good. So my point is this. To develop knowledge of the world we live in, we do have to rely on our sensory experience: what we see, what we touch, what we hear, taste, smell, and so on. But we have to be aware that our sensory experience is not totally reliable: it can deceive us. The conclusions that we draw from sensory experience are fallible. That means they might be wrong.
S: Can we stop here? I’ve got to pee.