LOGICAL REASONINGStudy Unit 5 – eLearning RPK 214

Cannot engage in logical

reasoning?

o Poor legal argumentso Loss of cases

And if you cannot win cases...

You MUST learn logical reasoning…

Base arguments on emotion /

feeling?

Convine your audience of your case, including your interpretation of the law

Solve legal problems

Test the acceptability of conclusions

Initial assumption (premise) Interim conclusions (inferences) Final conclusion

Rules of logic

PREMISE / STARTING ASSUMPTIONMUST BE TRUE / ACCEPTABLE

VALID INFERENCE / INTERIM CONCLUSIONMUST BE VALID

FINAL CONCLUSIONMUST FOLLOW LOGICALLY FROM THE ABOVE

If NOT follow logically: INVALID reasoning /

non sequitur

If DOES follow: argument SOUND!

PREMISE / STARTING ASSUMPTIONMUST BE TRUE / ACCEPTABLEAll men are liarsThapelo is a man

VALID INFERENCE / INTERIM CONCLUSIONMUST BE VALIDThapelo is a liar

FINAL CONCLUSIONMUST FOLLOW LOGICALLY FROM THE ABOVE

Premises = Untrue / unacceptable BUT: Conclusion = Valid – follows

logically from premises Argument is unsound

PREMISE / STARTING ASSUMPTIONMUST BE TRUE / ACCEPTABLE1: Fingerprints were found on the gun2: The fingerprints belong to Jimmy

VALID INFERENCE / INTERIM CONCLUSIONMUST BE VALIDJimmy’s finger touched the gun

FINAL CONCLUSIONMUST FOLLOW LOGICALLY FROM THE ABOVE

Premises = True / acceptable Conclusion = Valid – follows

logically from premises Argument is sound

PREMISE / STARTING ASSUMPTIONMUST BE TRUE / ACCEPTABLE1: Fingerprints were found on the gun2: The fingerprints belong to Jimmy

VALID INFERENCE / INTERIM CONCLUSIONMUST BE VALIDJimmy shot the victim

FINAL CONCLUSIONMUST FOLLOW LOGICALLY FROM THE ABOVE

?

PREMISE / STARTING ASSUMPTIONMUST BE TRUE / ACCEPTABLE1: Fingerprints were found on the gun2: The fingerprints belong to Jimmy

VALID INFERENCE / INTERIM CONCLUSIONMUST BE VALIDJimmy may have shot the victim

FINAL CONCLUSIONMUST FOLLOW LOGICALLY FROM THE ABOVE

Premises = True / acceptable Conclusion = Valid – follows

logically from premises Argument is sound

CONCLUSIVE INFERENCE

Jimmy touched the weapon

vs

PROBABILITY INFERENCE

Jimmy shot the victim

DEDUCTIVE REASONING

INDUCTIVE REASONING

DEDUCTIVE

• CONCLUSIVE INFERENCES

• Premise = TRUE

Inference = ALSO TRUE

• Argument = VALID (an argument is valid if conclusions follows premises)

• PROBABLE INFERENCES

• The higher the PROBABILITY, the more persuasive the

argument

Syllogism

INDUCTIVE

DEDUCTIVE LOGIC: THE SYLLOGISM

PREMISE 1: JOHN COMMITTED FRAUD

PREMISE 2: PEOPLE WHO COMMIT FRAUD ARE INTELLIGENT

CONCLUSION: JOHN IS INTELLIGENT

PROCESS OF REASONING / INFERENCE

PREMISE REASONING CONCLUSION

DEDUCTIVE LOGIC (cont.)

IF...

PREMISES = True / acceptable

PROCESS OF REASONING = Valid

•Final conclusions MUST BE ACCEPTED

•Argument is 100% concusive

BUT...

Practice of law...

Premises & inferences cannot always be proven to be 100% conclusive

INDUCTIVE LOGIC

Argument made using inductive reasoning

Prove argument by relying on different starting premises to prove argument = PROBABLY true

Proven on balance of probabilities

< persuasive than deductive reasoning

> persuasive than nothing!

degree of probability, persuasive effect

PREMISE 1: A group of 30 armed men is gathered outside the courtroom

PREMISE 2: Many members of the group are threatening to kill Bill

Conclusion: It is, therefore, PROBABLE that Bill’s life will be in danger if he is released on bail

INDUCTIVE LOGIC (cont.)

ATTACKING REASONING

PREMISE False / unacceptable

PROCESS OF REASONING / inferences drawn…flawed (Logical fallacy – causative / preconceptions / Tautologous)

Final conclusion does not follow series of inferences (non sequitur)

ATTACKING REASONING (cont.)

If you argue deductively & your opponent refuses to accept your premises...

Switch to inductive reasoning...

If fail: opponent must prove his assertions

ATTACKING REASONING (cont.)

When is premises true / accepted?

EVIDENCE!

Logical fallacies – When the evidence fails us

Causal fallacy Preconceived ideas Appealing to authority Tautologous arguments

ATTACKING REASONING (cont.)

DISTORTION OF EVIDENCEWhen receiving info always search for distortions in evidence

Omitting, understating, overemphasising facts

‘Surely’, ‘clearly’, ‘obviously’ substitute real evidence

Improper juxtaposition creates unfair impressions

Different interpretations of same evidence

ATTACKING REASONING (cont.)

IMORTANCE OF DEFINITIONS

Not understanding the correct meaning of words / concepts fuzzy thinking & fuzzy writing

Using words without understanding meaning properly opportunity for the opposition to attack reasoning on premise

Understanding words & concepts will allow YOU to attack opposition’s premises

Remember to complete class exercise 8…