PHIL 151 Week 8

Week 1 Propositional Logic

Propositional Logic Statement Premises (and hidden premises) Conclusion Argument Validity Truth Soundness contradiction

Propositional Logic Soundness Truth Validity Premises True premises Meaning (semantics) Are the premises true? Validity Argument Structure (syntax) the truth of the premises guarantees the truth of the conclusion. Soundness True premises

Propositional Logic Argument John is not going to class today because he is wearing shorts, and he never goes to class wearing shorts. Standard form John never goes to class wearing shorts. John is wearing sorts. -­‐-­‐ John is not going to class today.

Propositional Logic Hidden premises Felix is a cat. Cats hate birds. So, Felix hates Tweety. Felix is a cat. Cats hate birds. Tweety is a bird -­‐-­‐ Felix hates Tweety Cats hate birds

Week 2 Propositional logic

Propositional Logic Simple vs. complex statements Connectives (and, or, if, not) Truth Conditions Conditionals Argument forms

Logical connectives p q p and q p or q if p then q T F

Propositional Logic - Conditionals

Propositional Logic – propositional argument form I’ll have steak or a burger. If I have steak, I’ll feel full. If I have a burger, I’ll feel full. -- I’ll feel full. P: I’ll have steak Q: I’ll have a burger R: I’ll feel full P or Q If P then Q If Q then R -- R VALID

Week 3 Propositional Logic

2 ways in which a form is improper the argument is not a substitution It uses logical connectives

Truth tables – evaluating arguments A truth table is simply a table that represents all possible combinations of truth and falsity for the sentence letters & sentences in an argument. If P then Q If R then Q -- If P then R is there a row where the premises are true and the conclusion false?

Truth trees Another way of determining validity is by using truth trees. For complex arguments, truth trees are simpler than truth tables. Truth trees are based on decomposition rules for different logical connectives.

Truth trees

If P then Q If R then Q -- If P then R

Week 4

Lecture Categorical Statements – predicative logic  propositional logic Objects (individuals) Categories (predicates) Quantifiers (all, some, no) Universal (all) Existential (some) Logical equivalents (Converse, Obverse, Contraposition) Logical equivalent/ contraditory / contrary

Categorical argument All cats are ferocious No ferocious things are friendly. -- No cats are friendly. All A are B No B are C -- No A are C Valid

Categorical argument All C are M Cf -- Mf

Logical equivalents All birds have wings Converse: (change the order) All winged-things are birds Obverse: (change the value) No birds are non-winged things Contraposition: (change the value and the order) All non-winged things are non-birds

Circle diagram

All A are B No B are C -- No A are C A B C

All fish have gills. Nemo is a fish. So Nemo has gills. All F have G Fn Fn G -- Gn

8. No cats are vegans. No vegans eat meat. So all cats eat meat. No C are V No V eat M -- All C eat M C M V Invalid

Week 5 Meaning

Meaning Problems of meaning Extentional: all the individuals (universal) ostensive (one example, pointing out drawing) Enumerative (lists examples of term) Intensional: idea, or the concept of that individual Synonomous (give other terms with the same meaning Operational (describe a test to be used in applying a term) Problems of meaning Vagueness (Grey area) Ambiguity (the term has two meanings simultaneously) Equivocation (Same term appears each time with a different menaing)

Good definition? ‘Triangle’ means a three-sided figure.

Week 7 Fallacies

A Dozen Fallacies Come up with your own examples of each of the following fallacies, and then explain the flaw in your example. Equivocation Red Herring Slippery Slope Begging the Question Appeal to Pity Burden of Proof/No Nay-sayers Appeal to the People Ad Hominem Abusive, Circumstantial, and Tu Quoque Post-hoc ergo propter hoc No True Scotsman Texas Sharp-Shooter Straw Person

Evaluate the following argument for fallacies People should be able to play whatever games they like online. Playing an online game usually just means walking around and talking to other people, and in a free and democratic society, freedom of speech is a fundamental right. Some online games may seem violent, but since they’re just ‘games’, they aren’t truly violent: by definition, games are fun. If we don’t want to be victims of a moralistic crusade, we should stand up for freedom online. Red herring: “in a free and democratic society, freedom of speech...” Begging the question: “since they’re just ‘games’ they aren’t truly violent” Red herring: “by definition, games are fun”. (Could also be begging the question). Straw person: “If we don’t want to be victims...”