# Logical Proof and Deductive inference in classical and fuzzy logic

## Logical Proof:

Logical proof is an argument that establishes the validity of a proposition. Although proofs may be based on inductive logic, in general the term proof connotes a rigorous deduction. In

– Britanica

Logic is a way to quantitatively develop a reasoning process that can be replicated and manipulated with mathematical proofs

Logic is the ability of humans to reasonably think about something and make a decision based on some proofs

The proof is an argument from a hypothesis to a conclusion.

Each step of the argument follows the laws of logic.

- Analyze a given compound proposition which is given in the form of a linguistic statement
- Decompose this statement into single propositions
- Express the statement algebraically with all its logical connectives in the proper place
- Verify the truth value of the statement with a truth table

### Example: Logical proof

let us try to infer the conclusion from a given set of hypothesis

**Hypothesis:**

- Scientists are mathematicians
- Logical thinkers do not believe in magic
- Mathematicians are logical thinkers

**Conclusion:**

- Scientists do not believe in magic

**Proof:**

We will assign a capital letter to represent each hypothesis as a predicate.

- P: A person is a scientist
- Q: A person is a mathematician
- R: A person is a logical thinker
- S: A person believes in magic

Let us create a compound predicate by joining the individual predicates in the hypothesis using different connectives.

- Scientists are mathematicians: P â Q
- Logical thinkers do not believe in magic: R â S’
- Mathematicians are logical thinkers: Q â R
**Scientists do not believe in magic: P â S**‘

((P â Q) â§ (R â S’) â§ (Q â R)) â (P â S’)

We cant use the disjunction connective here because, in the case of disjunction, not all the statements in the hypothesis are required to prove the conclusion.

To check the truth of a given statement it is crucial to prove the entire hypothesis and the conclusion as true

## Proof by contradiction:

A statement is true only if its alternate statement is false

((P â Q) â§ P) â Q

((P’ â¨ Q) â§ P) â Q

(((P’ â¨ Q) â§ P))’ â¨ Q

Alternate statement: ((((P’ â¨ Q) â§ P))’ â¨ Q)’ = ((P’ â¨ Q) â§ P) â§ Q’

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## Implication:

The rule **IF A, THEN B**, i.e. (P â Q) = R = (A Ã B) âĒ (A’ Ã Y), is defined in function-theoretic terms as,

Ī_{R}(x, y) = maxâĄ((minâĄ(Ī_{A}(x), Ī_{B}(y))), min((1 – Ī_{A}(x)), 1))

The compound rule **IF A, THEN B, ELSE C **can also be defined in terms of a matrix relation as

R = (A Ã B) âĒ (A’ Ã C)

where the membership function is determined as

Ī_{R}(x, y) = maxâĄ((minâĄ(Ī_{A}(x), Ī_{B}(y))), min((1 – Ī_{A}(x)), Ī_{C}(y) ))

**Example:**

Suppose we have 2 universes for a thermistor problem and it is described by a collection of elements:

X = {1, 2, 3}, Y = {1, 2, 3, 4, 5}.

A crisp set A is defined on universe X and a crisp set B on universe Y as follows: A = {1, 3}, B = {2, 4, 5}

The inference given is **If A then B**: R=(A Ã B) âĒ (A’ Ã Y)

## Deductive Inference:

Also called **approximate reasoning**

**The ultimate goal of fuzzy logic **is to form the theoretical foundation for reasoning about imprecise propositions; such reasoning has been referred to as approximate reasoning [Zadeh, 1976, 1979].

Suppose we have a rule of the form **IF A, THEN B**, where A is a set defined in universe X and B is a set defined in universe Y. As discussed before, this rule can be translated into a relation between sets A and B

R = (A Ã B) âĒ (A’ Ã Y)

The **modus ponens deduction **is used as a tool for making inferences in rule-based systems. A typical ifâthen rule is used to determine whether an antecedent (cause or action) infers a consequent (effect or reaction).

Suppose a new antecedent, say **Aâ**, is known. Can we use modus ponens deduction to infer a new consequent, say **Bâ**, resulting from the new antecedent? That is, can we deduce, in rule form, **IF Aâ, THEN Bâ**?

Yes, through the use of the composition operation.

Bâ˛ = Aâ˛ â R = Aâ˛ â ((A Ã B) âĒ (A’ Ã Y))

## Test Your Knowledge:

**Hypothesis:**

- If you score 95% and above, your success is represented by a good grade
- 1. You score 95% and above

**Conclusion:**

- Your success is represented by a good grade

Can we derive the above conclusion from the given hypothesis using logical proof?

**Please post your answer / query / feedback in comment section below !**