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Home » People » Karl Sherlock » English 120 » Resources » info | Logic & Reasoning

Logic and Argument

Relevant exercise:

xrcs | Logic & Argument



appeal to intellect

  • clinical/technical tone

  • favoring facts and data over examples

  • organizing arguments overtly into logical syllogisms


"Let us begin with a simple proposition: What democracy requires is public debate, not information. Of course it needs information too, but the kind of information it needs can be generated only by vigorous popular debate. We do not know what we need to know until we ask the right questions, and we can identify the right questions only by subjecting our ideas about the world to the test of public controversy. Information, usually seen as the precondition of debate, is better understood as its by product. When we get into arguments that focus and fully engage our attention, we become avid seekers of relevant information. Otherwise, we take in information passively--if we take it in at all." (Christopher Lasch, "The Lost Art of Political Argument")


Deducting or Deducing: an act of taking away, or subtracting, leaving what remains

The use of syllogism to arrange a set of conditions or premises so that, through the act of logical subtraction, a definitive conclusion remains:

SYLLOGISM: an equation in which the truth of one premise and the true of another premise become a common truth: if p, then q; if q, then r: therefore, if p, then r.

Statements of "subtraction" and "conclusion" use the language of formal logic: if (when) . . . then; as a result; therefore; ergo; when . . . then; as a cause; in effect; etc.

An arrangement of premises:

MAJOR PREMISE: a general statement established as true and used as the basis on which to apply other premises

A = All men are mortals.

MINOR PREMISE: a more specific statement that relates to, or uses the language of, the Major Premise

B = Socrates is a man.

CONCLUSION: a specific assertion that is proven to be definitely or probably true and drawn from the Major and Minor Premise(s):

D = Therefore, Socrates is mortal.

Deductive Fallacies: errors in deductive reasoning caused by inconsistencies or errors in the way that the Major and Minor premises are presented, or in the methods of logic used to deduce the conclusions from them.

God is love, and love is blind. Stevie Wonder is blind, so Stevie Wonder is God.
  • Major Premise: God is love. 
  • Minor Premise: Love is blind. 
  • Conclusion: God is blind.
  • Major Premise: God is blind 
  • Minor Premise: Stevie Wonder is blind.
  • Conclusion: Stevie Wonder is God.

Though the above seems logically reasonable because it uses simple "A-to-B, B-to-C" reasoning, it is faulty in its logic on a number of levels.

  • Major Premise: All that is God [A] is love [B]. 
  • Minor Premise: All that is love [B] is blind [C]. 
  • Conclusion: All that is God [A] is blind [C].
  • Major Premise: All that is God [A] is blind [C]. 
  • Minor Premise: All that is Stevie Wonder [D] is blind [C].
  • (Distribution Error: not all that is Blind [C] is Stevie Wonder [D].)
  • Conclusion: All that is Stevie Wonder [D] is God [A].

Another problem with the above-mentioned syllogism is that it is guilty of EQUIVOCATION: the meaning of a key term changes or is used with multiple meanings. "Blind" does not mean physical blindness in the Major Premise and Minor Premise #1; rather, it is used as an analogy or a metaphor for the qualities of love that make one less aware of the faults in another. Therefore, the change in meaning from Minor Premise 1 to Minor Premise 2 shifts, and the conclusion is nonsensical. When the arguer intentionally shifts meanings without telling the audience or reader, this is said to equivocate, or "hide", the truth: it dissembles, and therefore is guilty of a lie by omission.

Other Deductive Fallacies

Begging the Question (petitio principii)
Since I'm not lying, it follows that I'm telling the truth.

Non sequitur (does not follow)
If the mill were polluting the river then we would see an increase in fish deaths. And fish deaths have increased. Thus, the mill is polluting the river.

Post Hoc, Ergo Propter Hoc (post hoc reasoning; coincidental correlation)
Ice Cream Rape Study

False Dilemma
America: love it or leave it.


Inducting or Inducing: an act of bring in, or bringing about, into the open; to persuade by inferring the general significance of specific evidence or examples.

In logic, inductive arguments develop from specific examples, details or facts into conclusions that are, both, more general and probably true (as opposed to definitive conclusions in Deductive Reasoning).

Q. How can you tell an elephant has been in your refrigerator?
A. There are elephant footprints in the butter.

The Specific Conditions are the evidence of footprints in the butter, from which a leap of inductive reasoning is made to assume that they can only be caused by an elephant.

Inductive Reasoning is a process of three major steps:

  • HYPOTHESIS: An intuitive conclusion or a speculative answer to a question.
  • EVIDENCE COLLECTION: An impartial example- or fact-gathering process
  • INFERENCE: a answer to the hypothesis based on what can be known from the evidence; a generalization about what's probably true (not possibly true, but probably)

Inductive Fallacies: errors in inductive reasoning caused by flawed hypotheses, flawed methodologies for evidence gathering, or flawed methods of inference

Hasty Generalization (Sweeping Generalization)
I asked six of my friends what they thought of the new spending restraints and they agreed it is a good idea. The new restraints are therefore generally popular.

Unrepresentative Sample (Misleading Facts)
To see how Canadians will vote in the next election we polled a hundred people in Calgary. This shows conclusively that the Reform Party will sweep the polls. (People in Calgary tend to be more conservative, and hence more likely to vote Reform, than people in the rest of the country.)

False or Weak Analogies
Government is like business, so just as business must be sensitive primarily to the bottom line, so also must government. (But the objectives of government and business are completely different, so probably they will have to meet different criteria.)

Slothful (Lazy) Induction:
Hugo has had twelve accidents in the last six months, yet he insists that it is just a coincidence and not his fault. (Inductively, the evidence is overwhelming that it is his fault.


image of puppy


appeal to emotion

poetical language and rhythms
allusions: literary and mythological
case examples, human interest 


For me, commentary on war zones at home and abroad begins and ends with personal reflections. A few years ago, while watching the news in Chicago, a local news story made a personal connection with me. The report concerned a teenager who had been shot because he had angered a group of his male peers. This act of violence caused me to recapture a memory from my own adolescence because of an instructive parallel in my own life with this boy who had been shot. When I was a teenager some thirty-five years ago in the New York metropolitan area, I wrote a regular column for my high school newspaper. One week, I wrote a column in which I made fun of the fraternities in my high school. As a result, I elicited the anger of some of the most aggressive teenagers in my high school. A couple of nights later, a car pulled up in front of my house, and the angry teenagers in the car dumped garbage on the lawn of my house as an act of revenge and intimidation. (James Garbarino "Children in a Violent World: A Metaphysical Perspective")


Fallacies of Emotion

Red Herring

Appeals to . . .

  • patriotism
  • fear
  • jingoism
  • like-ism
  • bandwagon

sad puppy 2


appeal to character (as opposed to "mores," which have to do with correct behavior)

  • sincere, caring about readers and the issues
  • trustworthy
  • truthful


"My Dear Fellow Clergymen:
. . . I think I should indicate why I am here in Birmingham, since you have been influenced by the view which argues against "outsiders coming in."...I, along with several members of my staff, am here because I was invited here. I am here because I have organizational ties here. But more basically, I am in Birmingham because injustice is here. Just as the prophets of the eighth century B.C. left their villages and carried their "thus saith the Lord" far beyond the boundaries of their home towns, and just as the Apostle Paul left his village of Tarsus and carried the gospel of Jesus Christ to the far corners of the Greco-Roman world, so am I compelled to carry the gospel of freedom beyond my own home town. Like Paul, I must constantly respond to the Macedonian call for aid. (Martin Luther King, Jr. "Letter from Birmingham Jail")

Fallacies of Authority

Ad hominem

Straw man

Doubtful Authority

Last Updated: 01/15/2016


Karl J. Sherlock
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