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Formal vs. Informal Logic
Logic
• The Art and Science of Reasoning
Formal Logic
• Formal Logic is about pure reasoning, it focuses
on deductive reasoning
– Whole to Part
• Leads you from your conclusion to your argument
• Deals with certainty, valid or invalid
• Types of arguments where the premise implies a
conclusion
– All Birds have wings
– A cardinal is a bird
– A cardinal has wings
Informal Logic
• Inductive Reasoning
– Part to Whole
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Probability
Focuses on arguing to a conclusion
Deals with shades of gray
Either strong or weak
Ordinary language argumentss
Fallacies
• A bad argument that fails to meet one of
these categories
– Relevance
– Clarity
– Presumption
Fallacies of Relevance
• Ad Fontem (to the source)
– Ad Hominem Abusive: Personal Attacks to the speaker
– Ad Hominem Circumstantial: Implies that the speaker
should not be trusted because of personal
circumstances
– Tu Quoque: The speakers advice should not be
followed because the speaker does not follow it
themselves
– Genetic Fallacy: an argument should be discounted
because of its source
Appeals to Emotion
• Appeal to fear (ad baculum): If you don’t agree with
me, bad things will happen
• Appeal to Pity (ad misericordium):Convince other of a
point by making them feel sorry for someone
• Mob Appeal (ad populum): Appeals to the common
man, to make up for lack of evidence
• Snob Appeal: To appeal to the rich or “cultured”
• Appeal to Illegitimate Authority (ad verecundium): an
attempt to shame the listener into agreement by citing
an illegitimate authority
• Chronological snobbbery: appeal to somethings age to
justify why it should be accepted
Red Herrings
• Appeal to Ignorance: because it can’t be
disproved, it is real
• Irrelevant Goals or Functions: because you
have unrealistic goals, you are unacceptable
• Irrelevant Thesis: Tying to prove something
that is irrelevant to the task at hand
• Straw Man Fallacy: an attempt to disprove
your opponent by presenting their arguments
in an inaccurate light
Questions to Ask
• Is it relevant and on topic?
• Is the argument leading to a conclusion that is
illegitimate?
• Is the Argument Clear?
• If not, it is a fallacy
Fallacies of Assumption
• Begging the Question- Assuming the
conclusion is true because the question is true
• False Dilemma- Assuming that the argument
has only 2 sides (Black and White)
• Fallacy of Moderation- Assuming that the
correct answer is always the middle of
extremes
• Is-Ought- Assuming that something IS a
certain way it SHOULD always be that way
Fallacies of Proof (Over Generalization)
• Sweeping Generalization- Making a Generalization
(that may be true) and applying it to all cases
• Hasty Generalization- Making a generalization about a
class of things on the basis of too few examples
• False Analogy: Making an analogy that is false or
inaccurate
• False Cause: Assuming that just because 2 things are
related, one caused the other
• Fake Precision: Uses numbers in a way that are far too
precise to be justified by the situation