![]() Inconsistent propositions are said to contradict one another. If there is no way for them all to be true at once, they are inconsistent. Two or more propositions are logically consistent if it is possible for them to all be true at the same time. On this page, we’ll consider three important logical relations: consistency, entailment, and equivalence. We can use similar methods to study the logical relations between propositions or sets of propositions. On the preceding pages, we saw how to use truth tables and the truth assignment method to determine whether arguments are valid or invalid, and to determine whether an individual proposition is a tautology, contradiction, or contingency. As seen above, ‘P v Q’ is a contingent statement – there are instances where it is true (row 1, 2 and 3), and an instance where it is false (row 4).Relations between Propositions: Consistency, Entailment, and Equivalence Notice on the first three rows of the table the claim is true, so it can’t be a contradiction.Ī contingent statement will have a truth table with both true and false rows. ‘P v Q’ is not a contradiction, as the following table shows: ‘P & ~P’ is a contradiction, as the following table shows: So, if there are any ‘T’s in the table, then the statement is not a contradiction. For a statement to be a contradiction, it has to always be false, so the table has to show all ‘F’s on the right side. Testing for contradiction works exactly opposite as testing for tautology. Even one F on the right side will mean that the claim is not a tautology (since there is at least one case in which it won’t be true). Notice that on row four of the table, the claim is false. ‘P v Q’ is not a tautology, as the following truth table shows: ‘P v ~P’ is a tautology, as this truth table shows: In other words, all Ts means that it is a tautology. If there are, then the statement is not a tautology. Since tautologies are always true, the way we test for them is to make a truth table for the statement and then to check every row of it to see if there are any Fs. ![]() It is also the case that these are the easiest things we can test for using tables, so it is a good place to start, even if ultimately, we don’t use the test very often. That said, sometimes claims will be very complex, and it may be less obvious which category they fall in. Certainly, this is true in the examples given here. We often say that tautologies are trivial, and contradictions are obvious. In all honesty, we don’t often need help determining if a sentence is a tautology, contradiction or contingency. If you have a cat, you won’t have mice.If a high pressure zone meets a low pressure zone, there’s be a tornado.If we go to the store, then we will buy some apples.It is raining right now, and it isn’t raining right now.Ĭontingencies, often called contingent statements, are true in some cases and not true in others. ![]() The following are examples of contradictions: There’s nothing you can do that can’t be done.Ĭontradictions are statements that are always false.Either it will rain tomorrow, or it won’t.The following are examples of tautologies: Tautologies are statements that are always true. When we are looking to evaluate a single claim, it can often be helpful to know if it is a tautology, a contradiction or a contingency.
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