MTH 181!
Rules of Inference V2!
1. Modus Ponens
2. Modus Tollens
p→q
p
∴q
p→q
q
∴ p
4. Specialization
a)
b)
p∧q
p∧q
p
q
∴q
∴p
5. Conjunction
7. Transitivity
8. Proof by Division into Cases
p∨q
p→r
q→r
∴r
p→q
q→r
∴p→r
Fall 2012
3. Generalizations
a)
b)
p
∴p∨q
q
∴p∨q
6. Elimination
a)
b)
p∨q
p∨q
q
p
∴p
∴q
p
q
∴p∧q
9. Contradiction Rule
 p→c
∴p
Example: Given the following, where are my glasses?
a) If I was reading the newspaper in the kitchen, then my glasses are on the kitchen table.
b) If my glasses are on the kitchen table, then I saw then at breakfast.
c) I did not see my glasses at breakfast.
d) I was reading the newspaper in the living room or I was reading the newspaper in the kitchen.
e) If I was reading the newspaper in the living room, then my glasses are on the coffee table.
Let RK = “I was reading the newspaper in the kitchen.”
GK = “My glasses are on the kitchen table.”
SB = “I saw my glasses at breakfast.”
RL = “I was reading the newspaper in the living room.”
GC = “My glasses are on the coffee table.
then the natural language statements above can be expressed as the following formal statements:
a’) RK → GK
b’) GK → SB
c’) ~ SB
d’) RK ∨ RL
e’) RL → GC
Now the following argument shows that my glasses are on the coffee table:
Step 1: Transitivity
RK → GK
GK → SB
∴ RK → SB
(a')
(b')
Step 2: Modus Tollens
RK → SB (1)
 SB
(c')
∴ RK
Thus, by Step 4 my glasses are on the coffee table!
Step 3: Elimination
RK ∨ RL (d')
 RK
(2)
∴ RL
Step 4: Modus Ponens
RL → GC (e')
RL
(3)
∴GC