Tautology

A tautology is a compound statement which is true for every value of the individual statements. The word tautology is derived from a Greek word where ‘tauto’ means ‘same’ and ‘logy’ means ‘logic’. A compound statement is made with two more simple statements by using some conditional words such as ‘and’, ‘or’, ‘not’, ‘if’, ‘then’, and ‘if and only if’. For example for any two given statements such as x and y, (x ⇒ y) ∨ (y ⇒ x) is a tautology.

The simple examples of tautology are;

  • Either Mohan will go home or Mohan will not go home.
  • He is healthy or he is not healthy
  • A number is odd or a number is not odd.

Tautology And Contradiction

We have already discussed the term tautology, which is true for every value of the two or more given statements. The contradiction is just the opposite of tautology or you can it contradicts the tautology statement. When a compound statement formed by two simple given statements by performing some logical operations on them, gives the false value only is called a contradiction or in different terms, it is called a fallacy. If (x ⇒ y) ∨ (y ⇒ x) is a tautology, then ~(x ⇒ y) ∨ (y ⇒ x) is a fallacy/contradiction.

We will also create a truth table here for better understanding the tautology and contradiction, but before that let us learn about the logical operations performed on given statements.

Logical Operations and Truth Tables

Logical Symbols are used to connect to simple statements, to define a compound statement and this process is called as logical operations. There are 5 major logical operations performed on the basis of respective symbols, such as AND, OR, NOT, Conditional and Biconditional. Let us learn one by one all the symbols with their meaning and operation.

AND Operation

AND is represented as ‘∧’ symbol. When two simple statements are used to form a compound statement using AND symbol, then it is called a conjunction of two statements.

Let x and y are two statements. See the table below to perform operation using AND symbol.

x

y

x ∧ y

T

T

T

T

F

F

F

T

F

F

F

F

OR Operation

OR is represented by ‘∨’ symbol. When two simple statements are used to form a compound statement using OR symbol, then it is called a disjunction of two statements.

x

y

x ∨ y

T

T

T

T

F

T

F

T

T

F

F

F

NOT Operation

When the truth value of a statement is changed using the word NOT, it is called as a negation of the given statement. It is denoted by ‘~’ symbol. Let x be a given statement then ~x is given by;

x

~x (NOT x)

T

F

T

F

F

T

F

T

Conditional Operation

When a compound statement is formed by two simple statements, connected with the phrase ‘if and then’, that is called conditional operation, where the conditional symbol is denoted by ‘⇒’. This symbol also denotes as implies.

x

y

x ⇒ y

T

T

T

T

F

F

F

T

T

F

F

T

Biconditional Operation

When a compound statement is formed by two simple statements, connected with the phrase ‘if and only if’, that is called biconditional operation, where the biconditional symbol is denoted by ‘⇔’. It also indicated as an equivalent symbol.

x

y

x y

T

T

T

T

F

F

F

T

F

F

F

T

Tautology Truth Table

Let x and y are two given statements. As per the definition of tautology, the compound statement should be true for every value.

x

y

x ⇒ y

y ⇒ x

Tautology = (x ⇒ y) ∨ (y ⇒ x)

Contradiction = ~(x ⇒ y) ∨ (y ⇒ x)

T

T

T

T

T

F

T

F

F

T

T

F

F

T

T

F

T

F

F

F

T

T

T

F

Tautology Examples

Example 1: Is ~h ⇒h is a tautology?

Solution: Given ‘h’ is a statement.

h

~h

~h ⇒h

T

F

T

F

T

F

Since, the true value of ~h ⇒h is{T,F}, therefore it is not a tautology.

Example 2: Show that p⇒(p∨q) is a tautology.

Solution:

p

q

p∨q

p⇒(p∨q)

T

T

T

T

T

F

T

T

F

T

T

T

F

F

F

T

The truth values of p⇒(p∨q) is true for all the value of individual statements. Therefore it is a tautology.

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