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Evidence and Arguments reasons for belief George Matthews CC 2017

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sources of belief

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sources of belief Why do we believe?

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sources of belief Why do we believe? ! Those around us believe it.

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sources of belief Why do we believe? ! Those around us believe it. ! We have heard it.

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sources of belief Why do we believe? ! Those around us believe it. ! We have heard it. ! We have seen evidence in favor of it.

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sources of belief Why do we believe? ! Those around us believe it. ! We have heard it. ! We have seen evidence in favor of it. Why should we believe?

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sources of belief Why do we believe? ! Those around us believe it. ! We have heard it. ! We have seen evidence in favor of it. Why should we believe? ! There is proof.

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sources of belief Why do we believe? ! Those around us believe it. ! We have heard it. ! We have seen evidence in favor of it. Why should we believe? ! There is proof. ! The evidence really shows it.

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sources of belief Why do we believe? ! Those around us believe it. ! We have heard it. ! We have seen evidence in favor of it. Why should we believe? ! There is proof. ! The evidence really shows it. ! It is the best explanation for the facts.

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sources of evidence

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sources of evidence 1. Personal experience

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sources of evidence 1. Personal experience 2. Eyewitness testimony

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sources of evidence 1. Personal experience 2. Eyewitness testimony 3. Expert opinion

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sources of evidence 1. Personal experience 2. Eyewitness testimony 3. Expert opinion 4. Research

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sources of evidence 1. Personal experience 2. Eyewitness testimony 3. Expert opinion 4. Research observational studies

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sources of evidence 1. Personal experience 2. Eyewitness testimony 3. Expert opinion 4. Research observational studies experimental studies

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what does the evidence show?

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what does the evidence show? ! Deduction

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows.

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs.

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs. ! Induction

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs. ! Induction Given this evidence, here is what is likely to be the case.

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs. ! Induction Given this evidence, here is what is likely to be the case. Good inductive arguments show what is highly probable.

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs. ! Induction Given this evidence, here is what is likely to be the case. Good inductive arguments show what is highly probable. ! Best-explanation

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs. ! Induction Given this evidence, here is what is likely to be the case. Good inductive arguments show what is highly probable. ! Best-explanation Given this evidence, here is the best way to explain it.

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what does the evidence show? ! Deduction Given this evidence, here is what logically follows. Successful deductive arguments are proofs. ! Induction Given this evidence, here is what is likely to be the case. Good inductive arguments show what is highly probable. ! Best-explanation Given this evidence, here is the best way to explain it. Best-explanation arguments are used to decide between competing explanations.

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal.

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. Premises: statements used to support conclusion.

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. Premises: statements used to support conclusion. Conclusion: what we are trying to establish.

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. To be really convincing an argument must be . . .

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. To be really convincing an argument must be . . . Valid

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. To be really convincing an argument must be . . . Valid Sound

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. To be really convincing an argument must be . . . Valid Sound Valid: “Can premises be true and conclusion false?” Sound: “Are premises really true?”

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. If this is true . . .

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. If this is true . . . and this is true . . .

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. If this is true . . . and this is true . . . This must be true.

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. This argument is VALID.

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deductive arguments All humans are mortal. Socrates was a human. Thus Socrates was mortal. It is also SOUND since premises are true.

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deductive arguments All lizards are warm-blooded. Socrates is a lizard. Thus Socrates is warm-blooded.

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deductive arguments All lizards are warm-blooded. Socrates is a lizard. Thus Socrates is warm-blooded. VALID but UNSOUND – both premises false.

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deductive arguments All dogs are mammals. My brother is a mammal. Thus my brother is a dog.

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deductive arguments All dogs are mammals. My brother is a mammal. Thus my brother is a dog. Suppose both premises are true . . .

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deductive arguments All dogs are mammals. My brother is a mammal. Thus my brother is a dog. Suppose both premises are true . . . conclusion could still be false.

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deductive arguments All dogs are mammals. My brother is a mammal. Thus my brother is a dog. Suppose both premises are true . . . conclusion could still be false. INVALID and so UNSOUND.

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deductive arguments All fish are priests. Trump is a priest. Thus Trump is a fish.

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deductive arguments All fish are priests. Trump is a priest. Thus Trump is a fish. Both premises are false.

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deductive arguments All fish are priests. Trump is a priest. Thus Trump is a fish. Both premises are false. But even if they were true conclusion could still be false.

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deductive arguments All fish are priests. Trump is a priest. Thus Trump is a fish. Both premises are false. But even if they were true conclusion could still be false. INVALID and so UNSOUND.

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argument analysis

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argument analysis identify argument

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argument analysis identify argument Is it valid?

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argument analysis identify argument Is it valid? Can premises be true and conclusion false?

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argument analysis identify argument Is it valid? Can premises be true and conclusion false? INVALID YES

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argument analysis identify argument Is it valid? Can premises be true and conclusion false? INVALID VALID NO

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argument analysis identify argument Is it valid? INVALID VALID Are premises true?

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argument analysis identify argument Is it valid? INVALID VALID Are premises true? no: UNSOUND discard

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argument analysis identify argument Is it valid? INVALID VALID Are premises true? no: UNSOUND UNSOUND discard

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argument analysis identify argument Is it valid? INVALID VALID Are premises true? no: UNSOUND UNSOUND discard yes: SOUND accept

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conditional reasoning test

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conditional reasoning test Which cards must you flip over to test the following rule?

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conditional reasoning test If it has an odd number on one side it has a vowel on the other.

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conditional reasoning test If it has an odd number on one side it has a vowel on the other. E M 3 6

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conditional reasoning test Whose ID or drink must you check to see if everyone is following the rule?

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conditional reasoning test If you are under 21 (X on hand), you cannot drink beer.

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conditional reasoning test If you are under 21 (X on hand), you cannot drink beer.

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conditional reasoning test If it has an odd number on one side it has a vowel on the other. E M 3 6

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conditional reasoning test If it has an odd number on one side it has a vowel on the other. E M 5 3 6

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conditional reasoning test If it has an odd number on one side it has a vowel on the other. E 5 C 6

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conditional reasoning test If you are under 21 (X on hand), you cannot drink beer.

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conditional reasoning test If you are under 21 (X on hand), you cannot drink beer.

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conditional reasoning test If you are under 21 (X on hand), you cannot drink beer.

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valid or invalid? More people will do crazy things tonight, because that’s what happens when the moon is full, and it is full tonight.

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valid or invalid? If the moon is full more people do crazy things. The moon is full tonight. So more people will do crazy things tonight. standard form

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valid or invalid? If the moon is full more people do crazy things. The moon is full tonight. So more people will do crazy things tonight. If A then B A B modus ponens

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valid or invalid? If the moon is full more people do crazy things. The moon is full tonight. So more people will do crazy things tonight. If A then B A B modus ponens VALID

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valid or invalid? If the moon is full more people do crazy things. The moon is full tonight. So more people will do crazy things tonight. If A then B A B modus ponens VALID UNSOUND

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Valid or invalid? If many children are not being vaccinated against measles, as is in fact the case, then measles out- breaks will occur. So we should expect that measles outbreaks will occur.

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Valid or invalid? If many children are not vaccinated against measles, measles outbreaks will occur. Many children are not being vaccinated against measles. Measles outbreaks will occur. standard form

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Valid or invalid? If many children are not vaccinated against measles, measles outbreaks will occur. Many children are not being vaccinated against measles. Measles outbreaks will occur. If A then B A B modus ponens

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Valid or invalid? If many children are not vaccinated against measles, measles outbreaks will occur. Many children are not being vaccinated against measles. Measles outbreaks will occur. If A then B A B modus ponens VALID

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Valid or invalid? If many children are not vaccinated against measles, measles outbreaks will occur. Many children are not being vaccinated against measles. Measles outbreaks will occur. If A then B A B modus ponens VALID SOUND

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valid or invalid? The earth is at the center of the universe. This is because if it were at the center, the sun would appear to travel across the sky, which it does.

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valid or invalid? If the earth were at the center of the universe, the sun would appear to travel across the sky. The sun does appear to travel across the sky. Thus the earth is at the center of the universe. standard form

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valid or invalid? If the earth were at the center of the universe, the sun would appear to travel across the sky. The sun does appear to travel across the sky. Thus the earth is at the center of the universe. If A then B B A affirming the consequent

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valid or invalid? If the earth were at the center of the universe, the sun would appear to travel across the sky. The sun does appear to travel across the sky. Thus the earth is at the center of the universe. If A then B B A affirming the consequent INVALID

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valid or invalid? If the earth were at the center of the universe, the sun would appear to travel across the sky. The sun does appear to travel across the sky. Thus the earth is at the center of the universe. If A then B B A affirming the consequent INVALID UNSOUND

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Valid or invalid? If you were abducted by aliens, you would remember it. Betty and Barney remember being abducted by aliens, which proves that they were in fact abducted by aliens.

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Valid or invalid? If you were abducted by aliens, you would remember it. Betty and Barney remember being abducted by aliens. Thus Betty and Barney were in fact abducted by aliens.

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Valid or invalid? If you were abducted by aliens, you would remember it. Betty and Barney remember being abducted by aliens. Thus Betty and Barney were in fact abducted by aliens. If A then B B A affirming the consequent

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Valid or invalid? If you were abducted by aliens, you would remember it. Betty and Barney remember being abducted by aliens. Thus Betty and Barney were in fact abducted by aliens. If A then B B A affirming the consequent INVALID

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Valid or invalid? If you were abducted by aliens, you would remember it. Betty and Barney remember being abducted by aliens. Thus Betty and Barney were in fact abducted by aliens. If A then B B A affirming the consequent INVALID UNSOUND

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Valid or invalid? If the earth were really moving, we would feel its motion. But we don’t, so it must not be moving.

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Valid or invalid? If the earth were moving we would feel its motion. But we do not feel the earth moving. So the earth must not be moving.

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Valid or invalid? If the earth were moving we would feel its motion. But we do not feel the earth moving. So the earth must not be moving. If A then B not B not A modus tollens or falsification

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Valid or invalid? If the earth were moving we would feel its motion. But we do not feel the earth moving. So the earth must not be moving. If A then B not B not A modus tollens or falsification VALID

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Valid or invalid? If the earth were moving we would feel its motion. But we do not feel the earth moving. So the earth must not be moving. If A then B not B not A modus tollens or falsification VALID UNSOUND

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Valid or invalid? If the Loch Ness Monster existed there would need to be hundreds of them. But there aren’t hundreds of them – if there were we would have seen them and we haven’t. So it does not exist.

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Valid or invalid? If the Loch Ness Monster existed there would need to be hundreds of them. But there are not hundreds of them. So it does not exist.

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Valid or invalid? If the Loch Ness Monster existed there would need to be hundreds of them. But there are not hundreds of them. So it does not exist. If A then B not B not A modus tollens or falsification

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Valid or invalid? If the Loch Ness Monster existed there would need to be hundreds of them. But there are not hundreds of them. So it does not exist. If A then B not B not A modus tollens or falsification VALID

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Valid or invalid? If the Loch Ness Monster existed there would need to be hundreds of them. But there are not hundreds of them. So it does not exist. If A then B not B not A modus tollens or falsification VALID SOUND

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Valid or invalid? Max isn’t a dog, since it is a fact that if it barks it’s a dog and Max didn’t bark.

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Valid or invalid? If it barks it is a dog. Max didn’t bark. So Max is not a dog.

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Valid or invalid? If it barks it is a dog. Max didn’t bark. So Max is not a dog. If A then B not A not B denying the antecedent

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Valid or invalid? If it barks it is a dog. Max didn’t bark. So Max is not a dog. If A then B not A not B denying the antecedent INVALID

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Valid or invalid? If it barks it is a dog. Max didn’t bark. So Max is not a dog. If A then B not A not B denying the antecedent INVALID UNSOUND

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Valid or invalid? She has no ESP because her zener card results were no better than we would expect from chance alone.

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Valid or invalid? If her zener card results were better than what we would expect from chance alone, she would have ESP. Her zener card results were no better than chance. So she does not have ESP.

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Valid or invalid? If her zener card results were better than what we would expect from chance alone, she would have ESP. Her zener card results were no better than chance. So she does not have ESP. If A then B not A not B denying the antecedent

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Valid or invalid? If her zener card results were better than what we would expect from chance alone, she would have ESP. Her zener card results were no better than chance. So she does not have ESP. If A then B not A not B denying the antecedent INVALID

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Valid or invalid? If her zener card results were better than what we would expect from chance alone, she would have ESP. Her zener card results were no better than chance. So she does not have ESP. If A then B not A not B denying the antecedent INVALID UNSOUND