This post is part of a series. To see the rest of the series, click here.
Rafa: Mom, Sandy’s Mom invited me for breakfast this Sunday. Can I go?
Samira: Who is Sandy?
R: She’s a new student in my school, in my class. She joined two weeks ago. Can I go?
S: Sure, we don’t have anything planned for Sunday.[A few minutes later.]
S: Rafa, why don’t you ask Sandy and her Mom for lunch one day? I’d like to meet them too.
S: Hmm. Let me see. How about lunch on 21st December 2509?
R: Is that going to be a weekend? Well, Sandy and I…. Mom!!! You said December 2509!
S: So I did.
R: But that’s ridiculous! We can’t have lunch in 2509.
S: Why not? Will you be fasting in 2509?
R: Be serious, Mom. We will all be dead by then.
S: How do you know we’ll all be dead by 2509? Did your doctor tell you? Or did an astrologer predict that we would all die by then?
R: *rolls eyes* We don’t need astrologers and doctors to tell us that. No human being can live for five hundred years.
S: Ah, so you’re making an inference.
S: Mm-hmm. Here is what you are saying. You know that no human being can live for five hundred years. You also know that you and I are human beings. And from these two statements, it follows that you and I won’t live for five hundred years, isn’t it?
R: You’re weird, you know that, Mom? Normal people don’t talk like you do.
S: Never mind my talking weird. Isn’t that what you meant?
R: Yes, I guess that’s true! So, that was an inference?
S: Yes. You inferred something from two premises. And you concluded something based on that inference. In this case, you concluded we cannot have lunch in 2509. But for that conclusion to be correct, those two premises should be correct. Right?
R: Premises? What’s that?
S: Premises are the starting points for an inference. You used two premises. One was that no human being can live for five hundred years. The second was that you and I are human beings. From these two premises, it follows logically that you and I won’t live for five hundred years. That was your conclusion, right? So, if the premises are correct, the conclusion has to be correct.
R: Oh, okay. I see it.
S: But what if one of the premises is wrong? Then we can’t be sure that the conclusion is right.
R: Huh? What do you mean?
S: How do you know that no human being can live for five hundred years?
R: How do I know that? It is true, isn’t it?
S: I am asking you on what basis you judge it to be true. Have you personally verified for the entire human population that every one of them dies before they are 500 years old?
R: No, but I have seen people dying before they are even a hundred years old, and I haven’t seen a single person who is more than a hundred years old, let alone 500 years old. And I haven’t heard of anyone like that. Has anyone ever reported coming across someone who is two hundred years old? If there was someone like that, somebody would have seen them.
S: Very good. That’s exactly what I was asking for. That is your justification for that premise.
R: Justification? Mom!!!
S: Justification is the reason or reasons you give to defend a conclusion, or an action. One of your premises was that no human being lives for five hundred years. I questioned that premise, and you gave me your justification for believing that one. That’s pretty good. But your conclusion is not completely certain, is it? Is there no chance that it can ever be wrong?
R: How can it be wrong?
S: Isn’t it possible — though extremely unlikely — that there are immortals among us, but they have carefully avoided being detected? One of the teachers from your school, for instance, may be an immortal for all you know, but she moves to a different town and different school before anyone can notice that she doesn’t grow old like other people.
R: It is extremely unlikely, but, yes, it is not impossible. I can’t disprove it. But what’s the point of all this, Mom?
S: Four points. One, many of our beliefs are actually conclusions. Two, these conclusions come from premises that we take for granted. Three, it’s important to make these premises explicit, and to question them. And four, if we question the premises, we discover that our beliefs, however strong, are not infallible.
R: Even the premise that you are a human being? Can even that belief be mistaken?
R: What, you are not a human?
S: I didn’t say that I am not human. All I said is that my belief that I am human and your belief that I am human can both be wrong, Same for our belief that you are human.
R: How can that belief be wrong?
S: Suppose you are an alien, and your parents exchanged you with the boy I delivered fifteen years ago without my knowing. Or maybe you’re a robot, and they exchanged my real son with you last week, without anyone’s knowledge. They implanted false memories in your robotic brain, so even you believe that you are my son. Aren’t these possible scenarios?
R: Now you’re talking Sci-fi. Possible, but not probable at all.
S: It doesn’t matter that they strike you as improbable. If you admit they are possible, and you can’t show that these possibilities are false, then you can’t be absolutely certain that you are a human being, right?
R: Okay. I guess so.
S: There are so many beliefs that we take for granted to be absolutely certain. When we examine them closely, most of them turn out to have at least some degree of uncertainty.
R: Hmm. I had never thought of that.
S: That’s okay, a first time for everything.
R: As usual, Mom, you took off in a completely different direction! When should we invite Sandy and her Mom for lunch? Given two possible conclusions, one that you and I will be alive on 21st December 2509, and the other that you and I won’t be alive on 21st December 2509, I’ve shown that the second belief is more reasonable. Haven’t I?
S: Yes indeed.
R: So, can I assume that you are a reasonable creature?
S: (laughing) A plausible, but not an absolutely certain assumption, yes.
R: When should we invite them for lunch?
S: How about next Sunday?
R: Sounds good.