What is Good Evidence? You are studying emerald samples from all over the world for your geology class. “This Columbian emerald is green…this South African.

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Presentation transcript:

What is Good Evidence?

You are studying emerald samples from all over the world for your geology class. “This Columbian emerald is green…this South African emerald is green … this Thai emerald is green…I’ve looked at samples of emeralds from 25 countries so far. They’re all green. I’m starting to think that ALL emeralds are green.” “I’m sorry,” your friend says, “Your samples actually show that all emeralds are GRUE.” “What are you talking about?” you ask. “Are you color blind?”

“Not at all. The evidence you have that emeralds are green is also evidence that emeralds are grue. An object is grue if it looks green prior to April 1, 2020, and blue after that date. You have just sampled emeralds from 25 countries and found that they were all grue.” “Well,” says your friend slyly, “What is there in what you see that tells you they will still be green after April 1, 2020?” “But that’s just crazy,” you say. “There’s nothing in what I see looking at these emeralds that tells me that they will be blue after April 1, 2020!”

An example of Deductive Logic : If the Premises of a good deductive argument are true, the conclusion must also be true. Such an argument is said to be valid. Premise #1: All bachelors are unmarried. Conclusion: Bill is unmarried. Premise #2: Bill is a bachelor.

An example of Inductive Logic : The Premises of a good inductive argument are good evidence that the conclusion is true. Nevertheless, the conclusion could be false. Premise #1: Emerald #1 from India is green. Conclusion: All emeralds are green. Premise #2: Emerald #2 from Thailand is green.. Premise #25: Emerald #25 from South Africa is green..

An example of Inductive Logic : However, if the samples come from many regions and many different geologic formations, we may find the evidence to be very compelling! Premise #1: Emerald #1 from India is green. Conclusion: All emeralds are green. Premise #2: Emerald #2 from Thailand is green.. Premise #25: Emerald #25 from South Africa is green..

An object is GRUE if it appears green before April 1, 2020, and blue after that date. All the observations we make which support the conclusion that emeralds are green provide equally good evidence that emeralds are grue!

An object is GRED if it appears green before April 1, 2020, and blue after that date. All the observations we make which support the conclusion that emeralds are green provide equally good evidence that emeralds are gred or grellow! An object is GRELLOW if it appears green before April 1, 2020, and yellow after that date.

An object is GRED2 if it appears green before April 2, 2020, and blue after that date. All the observations we make which support the conclusion that emeralds are green provide equally good evidence that emeralds are gred2 or grellow3! An object is GRELLOW3 if it appears green before April 3, 2020, and yellow after that date.

We can never have convincing evidence that a statement like “All emeralds are green” is true. We may make use of generalizations like “All emeralds are green” – we may order an emerald necklace to go with a green dress, assuming that the emeralds will be green. But we should always be prepared for our assumptions to turn out to be false. They might turn blue! Since inductive arguments are not deductively valid, we should not allow ourselves to be convinced by them.

The truth is what it is most useful to believe. We could reject deductively valid arguments if we found it useful to do so. And we can accept inductive arguments as conclusive if it is useful. We haven’t found it useful to believe that objects change color spontaneously. So we reject the conclusion that emeralds are grue. If it turned out to be useful to believe emeralds are grue, we reserve the right to reconsider!

Like a quality work of art, a quality conclusion is simple and elegant. We are guided by Quality when we conclude that emeralds are green, and reject the infinitely many alternative conclusion that they are grue, gred, etc. Nothing in our sensory experience shows us that objects are not grue or gred or grellow.

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