Arguments1
Deductive
-general principles, which are known, lead to specific conclusions
-conclusions are certain, inevitable
-usually derived from reason alone
-can be valid or invalid
-used by hard sciences
Types of Deductive Arguments:
1) Argument Based on Mathematics: an argument in which the conclusion depends on some
purely arithmetical or geometric computation or measurement. E.g. a shopper may place 2
apples and 3 oranges in her bag and then conclude that the bag contains 5 pieces of fruit.
2) Argument from Definition: an argument in which the conclusion is claimed to depend merely
on the definition of some word or phrase used in the premise or conclusion. E.g. someone might
argue that because Claudia is mendacious, it follows that she tells lies.
Inductive
-premise provides reasons supporting the probable truth of the conclusion
-reasons from specific cases to more general conclusions
-everyday arguments
-usually derived from experience
-conclusions are probable
-wide range of probability
Types of Inductive Arguments:
1) Argument from Analogy: an argument that depends on the existence of an analogy, or
similarity, between two things or states of affairs. E.g. Because Christina’s Porsche is a great
handling car, it follows that Angela’s Porsche must also be a great handling car.
2) Argument from Authority: an argument that is concludes something is true because a
presumed expert or witness has said that it is. E.g. a person might argue that earnings for HP
Corp. will be up in the coming quarter because of a statement to that effect by an investment
counselor.
3) Generalization: an argument that proceeds from the knowledge of a selected sample to some
claim about the whole group. E.g. 3 oranges selected from a certain crate were all juicy and
tasty, therefore all the oranges from that crate are especially juicy and tasty.
Central Ideas and Terminology Required to Evaluate Arguments
Valid: conclusion is entailed by its premise
Invalid: conclusion is not entailed by its premise
Sound: valid & the premises are true
Entailment: A fancy word for ‘follows from’
All men are mortals
Socrates is a man
Therefore, Socrates is a mortal
Valid and Sound
All 4-legged creatures have wings.
Spiders have exactly 4 legs.
1
Hurley, Patrick. (2008). A Concise Introduction to Logic. Belmont: Wadsworth.
Therefore, all spiders have wings
Valid and Not Sound (False Premise)
If I owned all the gold in Fort Knox I would be wealthy.
I do not own all the gold in Fort Knox.
Therefore, I am not wealthy
Invalid
Fallacies
Fallacy: a defect in an argument that consists in something other than a false premise alone
Deductive Fallacy: Deduction goes wrong, no longer a logical process; usually the premises are logically
irrelevant to the conclusion
1) Straw Man: committed when an arguer distorts an opponent’s argument for the purpose of
more easily attacking it, demolishes the distorted argument, and then concludes that the
opponent’s real argument has been demolished.
e.g. "Senator Jones says that we should not fund the attack submarine program. I
disagree entirely. I can't understand why he wants to leave us defenseless like that."
2) Argumentum ad hominem: (argument against the person) relies on attacking the person
taking the position.
e.g. “This is the best logical fallacy and if you disagree you smell.”
3) Argumentum ad Populum: (appeal to the people) appeals to popular belief
e.g. “Most people think this fallacy is the best, so clearly it is.”
Inductive Fallacy: occur because the connection between premises and conclusions is not strong
enough to support the conclusion.
1) Guilt by Association: Occurs when someone likes a specific idea or practice with something or
someone negative in order to infer guild on another person.
e.g. “Hitler was a vegetarian, therefore, I don’t trust vegetarians.”
2) Argumentum ad Ignorantiam: (appeal to ignorance) when the premises of an argument state
that nothing has been proved one way or the other about something, and the conclusion then
makes a definite assertion about that thing.
e.g. “You can’t prove that there aren’t Martians living in caves under the surface of
Mars, so it is reasonable for me to believe there are.”
3) Post hog ergo propter hoc: (after this, therefore because of this): an event is presumed to
have been caused by a closely preceding event.
e.g. “A black cat crossed my path and later I tripped and sprained my ankle. It must be
that black cats are really bad luck. “
4) Slippery Slope: occurs when the conclusion of an argument rests on an alleged chain reaction
and there is not sufficient reason to think that the chain reaction will actually take place.
e.g. “If we allow gay marriage what is next? Marriage between a human and a dog?”