formal logic n.
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Formal Logic

Formal Logic

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Formal Logic

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  1. Formal Logic Deductive reasoning

  2. Order and Design - Logic • Proverbs 4:7 • Proverbs 15:28 • Proverbs 18:17 • 1 Corinthians 14:33 • 1 Peter 3:15 • Jude 1:3 • Psalm 19:1 • Psalm 139:14

  3. Formal Logic is the science of deductive reasoning • Definition: “reasoning from known premises, or premises presumed to be true, to a certain conclusion.” • In contrast, most everyday arguments involve inductive reasoning. • reasoning from uncertain premises to probabalistic conclusions • “inference-making”

  4. Formal logic cannot establish the truth of the premises. The truth of the premises must be presumed, or taken as a given. Some premises may be proven or authenticated by scientific testing, reference to external sources, etc. Some premises may be granted or stipulated by all the parties to an argument Some premises may have been established as the conclusion of a previous argument DNA testing and paternity If a DNA sample is collected and analyzed properly and, If the DNA is an exact match with the alleged father, Then that person is the father. Structural validity versusmaterial truth

  5. There is no middle ground. A deductive argument can’t be “sort of” valid. By contrast, everyday arguments enjoy degrees of probability--plausible, possible, reasonable, believable, etc. In deduction, proofs are always valid or invalid.

  6. The form or structure of a deductive argument determines its validity • The fundamental property of a valid, deductive argument is that IF the premises are true, THEN the conclusion necessarily follows. • The conclusion is said to be “entailed” in, or contained in, the premises. • If all pigs have curly tails • And Nadine is a pig • Then Nadine has a curly tail

  7. The terms used in a syllogism must be defined precisely • If the meanings of key terms are vague or ambiguous, or change during the course of a deductive argument, then no valid conclusion may be reached. • Major premise: All pitchers hold water • Minor premise: Tom Glavin is a pitcher • Conclusion: Therefore, Tom Glavin holds water (the term “pitcher” has two different meanings in this argument, so no valid conclusion can be reached)

  8. Example of a valid deductive argument major premise: All cats have 9 lives minor premise: “Whiskers” is a cat conclusion: Therefore, Whiskers has 9 lives (Note: it doesn’t matter whether cats really have 9 lives; the argument is premised on the assumption that they do.)

  9. “Validity” versus “Soundness” • An argument is valid if its structure conforms to the rules of formal logic. • An argument is sound if it is valid, and its premises are true. • Thus validity is a prerequisite for soundness, but an argument needn’t be sound to be valid. • If sound, then valid too • If valid, not necessarily sound

  10. Example of a valid, but unsound argument major premise: All cats are pink minor premise: Felix is a cat conclusion: Therefore, Felix is pink (Cats aren’t pink, which makes the first premise untrue. Validity, however, presumes the truth of the premises.) Example of a valid and sound argument major premise: Anthrax is not a communicable disease minor premise: Communicable diseases pose the greatest threat to public health conclusion: Therefore, anthrax does not pose the greatest threat to public health (The premises are true and the conclusion is valid, that is, it necessarily follows from the premises) Validity versus soundness

  11. Syllogistic reasoning The syllogism is a common form of deductive reasoning. There are different types of syllogisms categorical (universal premises) hypothetical (if-then premises) disjunctive (either-or premises) All follow the basic form: major premise minor premise conclusion

  12. Categorical syllogisms rely on universal premises Example of a valid categorical syllogism: major premise: All Christians believe Jesus is the son of God. minor premise: Bill is a Christian. conclusion: Bill believes Jesus is the son of God.

  13. Hypothetical syllogisms use “if-then” premises Example of a valid hypothetical syllogism: Major premise: If Biff likes Babbs, then he’ll ask her to the prom. Minor premise: Biff likes Babbs, Conclusion: Therefore, he’ll ask her to the prom.

  14. Disjunctive syllogisms use “either-or” premises Example of a valid disjunctive syllogism: Major premise: Either Babbs will get her navel pierced, or she’ll get a tongue stud. Minor premise: Babbs didn’t get her navel pierced. Conclusion: Therefore, Babbs got a tongue stud.

  15. Mistakes in reasoning – Logical Fallacies • Reification • Equivocation • Begging the Question • Question-begging Epithet • Complex Question • Bifurcation • Ad Hominem • Faulty Appeal to Authority • The Strawman Fallacy • Formal Fallacies

  16. Reification • Reification is the attributing a concrete characteristic to something that is abstract. • It’s not nice to fool mother nature. • This is an example of reification because “nature” is an abstraction; it is simply a name we give to the chain of events in the universe. Nature is not a person and cannot be fooled.

  17. Not all language should be taken literally • There is nothing wrong with reification as a figure of speech. • Even the Bible uses reification at times in its poetic sections like Proverbs 9 where the Bible personifies the concept of wisdom.

  18. Fallacies • When reification is used as part of a logical argument, it is a fallacy. • Using such a poetic expression is often ambiguous and can obscure important points in a debate.

  19. Fallacies of Reification • “Nature has designed some amazing creatures.” • This sentence commits the fallacy of reification because nature does not have a mind and cannot literally design anything. By using the fallacy of reification, the evolutionist obscures the fact that the evolution worldview really cannot account for the design of living creatures.

  20. Fallacies of Reification - continued • God can design creatures because God is a supernatural being. • Nature is a concept and cannot design anything.

  21. Fallacies of Reification - continued • “Creationists say the world was created supernaturally, but science says otherwise.” • Here the person has attributed personal, concrete attributes to the concept of science. • In doing so, he or she overlooks the important fact that scientists draw conclusions about the evidence and verbalize such conclusions – not “science.”

  22. Fallacies of Reification - continued • Science is a conceptual tool that can be used properly or improperly. It says nothing. It does not take a position on issues. So this common example of reification is logically fallacious.

  23. Fallacies of Reification - continued • “The evidence speaks for itself.” • This expression is quite common, but when used as part of an argument, it is the fallacy of reification. • Evidence does not speak at all. Evidence is a concept: the name we give to a body of facts that we believe to be consistent with a particular point of view. People draw conclusions about evidence and verbalize their thoughts.

  24. Fallacies of Reification - continued • “Evolution figured out a way around these problems.” • A number of evolutionists have said something along these lines when attempting to explain some intricately designed biological system. • Evolution is a concept. It has no mind and cannot figure out anything. So this example again obscures the difficulty in accounting for design in the universe without appealing to the mind.

  25. Fallacies of Reification - continued

  26. Fallacies of Reification – Natural Selection • “Natural Selection.” • This phrase is an example of reification and could be considered a fallacy, if used in an argument. Nature cannot literally select. • The phrase is so commonly used that we might not call it a fallacy, providing the meaning is understood by all.

  27. Fallacies of Reification – Natural Selection • We do believe in the concept called “natural selection.” Yes, organisms that are well-suited to an environment are more likely to survive than those that are not well suited. • This is tautologically true, a statement always considered correct, and is something that both creationists and evolutionists believe.

  28. Fallacies of Reification – Natural Selection • Suppose we ask, “Why is it that animals are well-suited to their environment?” • If an evolutionists answered “natural selection,” this would be the fallacy of reification. • It poetically obscures the true reason that animals are designed to survive – God.

  29. Fallacies of Reification – Natural Selection • If you think about it, natural selection does not actually explain why we find organisms suited to their environment. • It only explains why we do not find organisms unsuited to their environment (i.e., because they die). • It is God – not “nature” – who has given living beings the abilities they need to survive.

  30. Fallacies of Reification – Conclusion • Often the concept being reified is given personal characteristics: the ability to think, to have an opinion, and so on. • When concepts are personified in this way in an argument, it is sometimes called the “pathetic fallacy.” • From the word empathy because we are attributing thoughts and feelings to something that cannot possess them.

  31. Examples of Reification • “Nature has found a way.” • “Life invaded the dry land.” • “Natural selection guided development of this species.” • “Science say that we must limit explanations to the natural world.” • “Follow the evidence where it leads.” • Evolution tells us much about the way the world works.”

  32. Is God good?

  33. Logic Equivocation

  34. Equivocation – (bait & switch) • When someone shifts from one meaning of a word to another within an argument, he or she has committed the fallacy of equivocation. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 123-124). Master Books. Kindle Edition.

  35. Equivocation - example • “Doctors know a lot about medicine, and Dr. Lisle is a doctor. So he must know a lot about medicine.” • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 124-125). Master Books. Kindle Edition. Medical Doctor Jason Lisle, Phd.

  36. Equivocation - example • This use of equivocation is sometimes called a “bait and switch” fallacy because the listener is baited on one meaning of a word, and then the meaning is switched to draw a faulty conclusion. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 126-127). Master Books. Kindle Edition.

  37. Equivocation – bait and switch

  38. Equivocation - evolution • Evolutionists often commit the fallacy of equivocation on the word evolution. This word has a number of meanings. • Evolution can mean “change” in a general sense, but it can also refer to the idea that organisms share a common ancestor. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 128-129). Master Books. Kindle Edition.

  39. Equivocation - evolution • Many evolutionists seem to think that by demonstrating evolution in the sense of “change,” it somehow proves evolution in the sense of “common descent.”

  40. Fallacies of Reification - continued • You might hear them say something like, “Creationists are wrong because we can see evolution happening all the time. Organisms are constantly changing and adapting to their environment.” But, of course, the fact that animals change does not demonstrate that they share a common ancestor. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 130-133). Master Books. Kindle Edition.

  41. Equivocation - evolution

  42. Equivocation • I cannot overstate how common this fallacy is in evolutionary arguments. Bacteria becoming resistant to antibiotics, speciation events, changes in the size and shape of finch beaks, the development of new breeds of dog, and changes in allele frequency are all examples of change, but none of them demonstrate that the basic kinds of organisms share a common ancestor. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 133-136). Master Books. Kindle Edition.

  43. Equivocation - science • Science commonly refers to the procedures by which we explore the consistent and predictable behavior of the universe today — the scientific method. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 139-140). Master Books. Kindle Edition. • This is also know as observational science.

  44. science can also refer to a body of knowledge (e.g., the science of genetics). Furthermore, science can also refer to models regarding past events; this is origins science. Or it can refer to a specific model. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 140-142). Master Books. Kindle Edition. • Scientific models regarding past events is also know as historical science.

  45. Equivocation – science • “Science has given us computers, medicine, the space program, and so much more. Why then do you deny the science of evolution?” • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 142-143). Master Books. Kindle Edition. • This argument merges observational science with historical science. Historical science lacks the testable, repeatable aspects of observational science because the past can never be tested or repeated.

  46. Equivocation • By merging operational science with evolution, the arguer hopes to give evolution a credibility that it does not truly deserve. Yes, we do believe in operational science, and we have some respect for origins science as well. However, this does not mean that we should believe in evolution — which is only one particular model of origins science. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 146-148). Master Books. Kindle Edition.

  47. Equivocation Hey! You can’t just take that! It’s stealing! So what? They steal in baseball all the time. If it’s ok to for baseball players to steal, logically it’s ok for me to steal too.

  48. Equivocation - interpretation • Old-earth creationists often commit this fallacy on the word interpretation. They may say, “We must always compare our interpretation of Scripture with our interpretation of nature.” Interpretation of the Scripture means to understand the meaning of the propositional statements — to grasp the author’s intention. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 148-150). Master Books. Kindle Edition.

  49. Equivocation - interpretation • However, nature does not have intentions. When we interpret nature, we are creating propositional statements about nature. • By merging these two meanings of interpretation, the old-earth creationist places scientists’ statements about nature on the same level as Scripture. • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 150-151). Master Books. Kindle Edition.

  50. Equivocation – examples • “Science is a very powerful tool, so why deny the science of evolution?” • “Evolution is a scientific fact. The evolution of bacteria becoming resistant is well-documented.” • Lisle, Jason (2010-07-01). Discerning Truth (Kindle Locations 153-155). Master Books. Kindle Edition.