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 actualy true, only the question of whether or not the things we are assuming are sufficient to conclude some other property.
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 finite numbers of 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 conclusionQ(writtenP₁, P₂, ... ⊧ Q) if, for all valuationsv, whenever all the assumptions are true underv(i.e., for alli,〚P_i〛v = T), then the conclusion is true underv(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.
The following video introduces the idea of entailment by examples.
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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.
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) |
|---|---|---|
| F | F | F |
| T | T | T |
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.
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) |
|---|---|---|---|
| F | F | F | F |
| T | F | F | T |
| F | T | F | F |
| T | T | T | T |
Again, we check each valuation (row) to see whether whenever all the assumptions are true, the conclusion is true. For this table:
- In rows 1 and 3: the assumption
A \land Bis false, so it does not matter that the conclusion is false. - In row 2: the assumption is false and the conclusion is true. Since the assumption is false, we do not actually care what the conclusion is.
- In row 4: the assumption is true, so we need to check the conclusion. The conclusion is also true, so this row is good.
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.
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