# Prove that the following is true: a sentence is unsatisfiable if and only if it implies all other sentences.

The issue is with attempting to prove what the title mentions. I am told that the first part of my proof makes sense:

PROOF

An unsatisfiable sentence is false in all of its interpretations

Suppose A is an unsatisfiable sentence and B is any sentence

so, A is false in all of its interpretations

There is no interpretation in which A is true and B is false

A implies B

Now the issue I am having is continuing this proof and refining the second part to it (below) in order to make it make sense:

PROOF cont.

Now suppose B implies A

There is no interpretation where B is true and A is false

For all interpretations where A is false, B is false

A is false in all of its interpretations

so, B is false

so, Any sentence B implies an unsatisfiable sentence A

END PROOF

So the frist direction of the "if and only if" statement is done i.e. the direction where you assume that $A$ is not satisfiable. So for the second direction, why do you assume that B implies A? Especially, you can not here assume that A is not satisfiable (as this is what you want to prove) which you actually do on row 4 in the second part.

Instead for the second direction, assume that a sentence $A$ implies all other sentences. So especially it implies $\neg A$. Thus $A\rightarrow \neg A$ hold. However if $A$ is ever interpreted as true in any evaluation, this implication does not hold. Thus we conclude that $A$ allways needs to be interpreted as false i.e. $A$ is unsatisfiable.

• Would this be correct for the second part then: Suppose a sentence A implies any sentence B. So, there is no interpretation where A is true and B is false. So, For all interpretations where A is true, B is true. now, suppose B is a sentence that is the negation of A. So, there is no interpretation where A is true. For all interpretations where A is false, -A is true and the implication holds. Conclusion: So, any sentence implies any unsatisfiable sentence. Does that make any sense? – SeesSound Feb 25 '16 at 14:49
• Yes, that seems pretty good except you can stop at "there is no interpretation where A is true", as this is what it means for A to be unsatisfiable, thus you are done. – Ove Ahlman Feb 25 '16 at 14:56