CS208 Logic & Algorithms

Semester 1: Logic

Entailment

One of the uses of logic (and possibly the most important one) is to identify when conclusions do and do not follow from some assumptions.

Logic does not concern itself with whether or not whether or not the assumptions are actually true, only the question of whether or not the formulas we are assuming are sufficient to conclude some other formula.

If a conclusion Q follows from some assumptions P₁, P₂... (in a rigorous way we define below), we will say that the assumptions P₁, P₂... entails Q. I will also write this in symbols:

  P₁, P₂ ... ⊧ Q

The assumptions P₁, P₂... may be finitely or infinitely many, though we will usually only be interested in finitely many assumptions.

The intuitive definition of entailment is the following:

If all the assumptions P₁, P₂, ... are true, then the conclusion is true.

To make this formal, we must be explicit about valuations. So the full definition of entailment is:

Assumptions P₁, P₂... entail a conclusion Q (written P₁, P₂, ... ⊧ Q) if, for all valuations v, whenever all the assumptions are true under v (i.e., for all i, 〚P_i〛v = T), then the conclusion is true under v (i.e., 〚Q〛v = T).

The definition of entailment can be subtle and difficult to understand at first without working through a few examples, so I have written these up below.

Video

The following video introduces the idea of entailment by examples.

Slides (PDF)

Example Entailments

For Propositional Logic, it is possible to compute entailments by using truth tables. Let's see how to do this by some examples. These examples will also introduce some of the interesting consequences of the definition of entailment.

“A” entails “A”

Our first example is the entailment A \models A. Inituitively, this entailment ought to hold: if we assume A, then we should be able to conclude A. Let us check this by writing a truth table. We first write out all the possible values of A. These are all our different valuations, so we explicitly mark these columns as our valuation:

A (valuation)...

Then we write down our assumptions as further columns and fill in the truth values. If our assumptions are composed of multiple connectives, we might need to put in extra columns for intermediate working. In this case, we only have one assumption which is just A, so we can write down directly:

A (valuation)A (assumption)...

To complete the table, we compute the values of the conclusion for each valuation. Again, this might require some intermediate working if the conclusion formula is complicated. in this case, it is just A, so we can write it down directly and fill out the rows in the table for all possible values of A:

A (valuation)A (assumption)A (conclusion)
FFF
TTT

Now, to check the entailment we have to check: for every valuation (row), if all the assumptions are true, then the conclusion is true. If this holds in every valuation (row), then entailment is valid. Otherwise, it is invalid.

Checking this table, we can see that the entailment holds. In the first row, the assumption is false, so we are OK. In the second row, the assumption is true, but so is the conclusion, so we are OK. After checking every row, we can conclude that A \models A.

“A and B” entails “A”

The entailment A \land B \models A is a little more complex because there are two propositional atoms A and B. I will write out the table all in one go:

A (valuation)B (valuation)A \land B (assumption)A (conclusion)
FFFF
TFFT
FTFF
TTTT

Again, we check each valuation (row) to see whether whenever all the assumptions are true, the conclusion is true. For this table:

Together, for each row, we have that if the assumption is true, then the conclusion is true. Therefore, the entailment A \land B \models A is valid.

“A or B” does not entail “A”

TBD

A (valuation)B (valuation)A \lor B (assumption)A (conclusion)
FFFF
FTTF
TFTT
TTTT

Implication internalises entailment

TBD

A (valuation)B (valuation)A \to B (assumption)A (assumption)B (conclusion)
FF
FT
TF
TT

Entailment with no assumptions

TBD

Contradictory assumptions

TBD

A (valuation)B (valuation)A (assumption)¬A (assumption)B (conclusion)

Facts about entailment

Substitution

TBD

Order does not matter

TBD

Comma means “And”

TBD

Adding assumptions does not matter

TBD

Contradictory assumptions entail everything

TBD

The Deduction Theorem

TBD

Relationship between Satisfiability, Validity and Entailment

TBD