Lecture Two: The Nature of Logic and Reasoning II
September 14, 2011
Recap of Last Lecture
Reasoning or argument is the type of discourse which tries to support a point by
appealing to evidence.
The point it tries to support is the conclusion and the evidence appealed to is stated in
Justice vs. Fairness
Are justice and fairness the same thing? Do they overlap at some point or are they two
completely different concepts?
Types of Arguments
Definition - arguments in which IF the premise or premises is/are true, then the
conclusion MAY be true by virtue of this.
You may assume anything that is not contradictory, even though they may not be
P1 - This piece of iron sinks in water
P2 - That piece of iron sinks in water
C - All pieces of iron will sink in water
There is a large leap between what was observed between the two pieces of iron and
what occurs to ALL pieces of iron.
This conclusion is believed to be true, although it will never be empirically proven
because nobody will ever have observed all pieces of iron placed into water to see what
A stronger inductive argument has more proofs. For example, if someone observed 2
pieces of iron vs someone who observed 200 pieces of iron. The second argument
would be stronger because more were observed. Example:
P1 - My first former husband, who was a hard worker, was no good.
P2 - My second former husband, who was a banker, was no good.
P3 - My third former husband, who was a world famous actor, was no good.
C - No men are good.
This is an inductive argument because the premises are assumed to be true because
they do not contradict one another, therefore the conclusion makes sense (based on the
examples given) even though it may not be true.
Definition - arguments in which the truth of the conclusions either follows necessarily
from the premise/s IF it/they is/are true or necessarily does not follow form the premise/s
EVEN IF it/they is/are true.
There are valid and invalid deductive arguments.
P1 - All pieces of iron will sink in water.
C - This piece of iron sinks in water.
In this case, the premise is assumed to be true however it cannot be proven whether or
not it truly is. The conclusion follows the premise assuming it is true.
P1 - 2=(1+1)
P2 - 4=(1+1)+(1+1)
P3 - 2+2 = (1+1)+(1+1)
C - 2+2=4
This is the best type of deductive argument because none of the premises can be
disputed or questioned.
P1 - My cat is Felix
P2 - My friends cat is Felix
P3 - My friends friends cat is Felix
C - My cat, my friends cat and my friends friends cat are all Felixes
This is true assuming that all the p