Patterns of Informal Non-Deductive Logic (Ch. 6)

Non-Deductive Arguments 1 Chapter 6

Non-Deductive Arguments Non-Deductive /Inductive arguments: If the premises are true, the conclusion is NOT necessarily/guaranteed true

Non-Deductive Arguments Non-deductive arguments cannot give proofs All non-deductive arguments are invalid, but there are good and bad ones Non-deductive arguments fall on a scale from strong to weak Strong: a non-deductive argument where the conclusion is probably true assuming the premises are true Weak: a non-deductive argument where the conclusion is probably not true assuming the premises are true Cogent argument: strong argument AND premises are true Cogent arguments are good non-deductive!

Overview Deductive Arguments Non-Deductive Arguments Sound Cogent Stronger Weaker Valid Invalid Valid Arg: sound or unsound Invalid Arg: always unsound Strong Arg: cogent or uncogent Weak Arg: always uncogent

Kinds of Non-Deductive Arguments 1 Chapter 6

Inductive Generalization Generalization: premises about particulars, general conclusion. Example: Inductive Generalization One swan is white Another swan is white … So, all swans are white This is a cogent argument: strong with true premises strong because the conclusion is probable assuming the premises are true the premises are all true

Revising Inductive Generalizations For centuries in Europe, it was thought that there were only white swans. So, the above argument looked like it produced a true conclusion. Later, black swans were discovered in Australia. So, in fact, the conclusion is false. However, the argument is still a cogent one (strong with true premises) We can revise the argument now to include the new evidence and give a stronger argument One swan is white Another is white Another is black … So, all swans are black or white

Induction but not a generalization Induction can also have a particular conclusion (about a specific thing not all) The textbook calls some of these “arguments from past experience” Example One swan is white Another swan is white … That one swan over there by the lake is white

Faulty Inductive Generalization Inductive Generalizations are faulty when i) there are not enough samples (i.e. particular premises) For example, if we concluded that all swans are white after observing two swans, the sample size would be way too small. ii) the samples are not representative For example, if we looked at swans that we covered in blue paint at a circus. Even if we observed many cases of blue swans at a circus, the sample would not be a representative one.

Argument by Analogy Argument by analogy: two things are similar in some respects, so they are similar in some other respect A has characteristic W, X, Y B has characteristic W, X, Y A has characteristic Z So, B probably has characteristic Z Example Earth has oxygen, hydrogen and water Mars has oxygen, hydrogen and water Earth has life So, Mars has life

Fallacy of Weak Analogy When the characteristics compared in premises are irrelevant to characteristic of conclusion Example Child pornography is sold and distributed Newspapers are sold and distributed Child pornography should be outlawed So, newspapers should be outlawed

3 Views on the Existence of God Theism: I know that God exists: I have a good argument that God exists Atheism: I know God does not exist: I have a good argument that God doesn’t exist Agnosticism: I don’t know whether God exists: I don’t have a argument either way. Arguments One argument by analogy for theism: The argument from design One deductive argument for atheism: the problem of evil One inductive argument for atheism: the problem of evil

Design Argument for the Existence of God The Argument from Design A watch is finely-tuned, complex, So a watch has a designer The Universe is finely-tuned, highly complex. So, the world has a designer by a designer (God) Objection: the watch and the world are not remotely similar. We know just about everything about how a watch works (e.g. what happens when you drop it in watter, what its parts do, etc.), but we know almost nothing about the Universe (how did it start, can it be destroyed, does it end, etc.). Because the world and the watch are so very different we cannot make the analogy

Deductive Argument that God Doesn’t Exist The Problem of Evil Argument Suppose God exists Hypothesis God knows about all evil that has or will happen God can do anything God is good: he eliminates as much evil as he knows about and can So, God knows about all evil and eliminates all evil. from 1-3 So, there is no evil in the world from A,5 7 But, there is evil in the world (earthquakes, WW2) Premise Since 5 and 6 contradict, A. must be false So, God does not exist

Objection The Problem of Evil Argument is a proof that God doesn’t Exist; It shows that any evil is impossible if God exists, because he would have prevented it from happening Objection: God gave us freedom He does not help us because he wants us to make our own choices. He knew about WW2, and Indonesian Tsunami but let it happen anyway because it is better for humans to make their own choices (to be tested and deal with loss) than to have a world where humans do not make free choices Premise 6 is false: Earthquakes, WW2 are not evils.

Inductive Argument from Evil Many are convinced by “Freedom Objection” to the Deductive Argument. There are other, non-deductive arguments that God does not exist. Even if we assume that God gave us freedom, why must WW2 have happened or the Indonesian Tsumani or many other similar events. This argument collects as evidence those cases and argues that probably God does not exist The Inductive Argument from Evil Indonesian Tsunami is an evil preventable for God WWII is an evil preventable by God … …. So, God does not exist