Language, Proof and
Logic
Formal Proofs and
Boolean Logic
Chapter 6
6.1
Conjunction rules
 Elim:
 Intro:
P1…Pi…Pn
P1

Pn
…
P1…Pn
…
Pi
1. ABC
2. B
3. C
4. CB
 Elim: 1
 Elim: 1
 Intro: 3,2
6.2a
Disjunction rules
 Intro:
 Elim:
Pi
P1…Pn
…
…
P1…Pi…Pn
P1
…
S

Pn
…
S
…
S
6.2b
Example
1. (AB)  (CD)
2. AB
3. B
4. BD
 Elim: 2
 Intro: 3
5. CD
6. D
7. BD
8. BD
 Elim: 5
 Intro: 6
 Elim: 1, 2-4, 5-7
You try it, page 152
6.3
Contradiction and negation rules
 Elim:
 Intro:

…
P
…
P
…

P
 Elim:
P
…
P
 Intro:
P
…

P
You try it, p.163
6.4
The proper use of subproofs
A subproof may use any of its own assumptions and derived sentences,
as well as those of its parent (or grandparent, etc.) proof.
However, once a subproof ends, its statements are discharged. That is,
nothing outside that subproof (say, in its parent or sibling proof) can cite
anything from within that subproof.
6.5
Strategy and tactics
When looking for a proof, the following would help:
1. Understand what the sentences are saying.
2. Decide whether you think the conclusion follows from the premises.
3. If you think it does not follow, look for a counterexample.
4. If you think it does follow, try to give an informal proof first, and
then turn it into a formal one.
5. Working backwards is always a good idea.
6. When working backwards, though, always check that your
intermediate goals are consequences of the available information.
You try it, page 170.
6.6
Proofs without premises
The conclusion of such a proof is always logically valid!
1. PP
2. P
3. P
4. 
5. (PP)
 Elim: 1
 Elim: 1
 Intro: 2,3
 Intro: 1-4