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Inference in First-Order Logic

Inference in First-Order Logic is used to deduce new facts or sentences from existing sentences. Before understanding the FOL inference rule, let's understand some basic terminologies used in FOL.


Substitution is a fundamental operation performed on terms and formulas. It occurs in all inference systems in first-order logic. The substitution is complex in the presence of quantifiers in FOL. If we write F[a/x], so it refers to substitute a constant "a" in place of variable "x".

Note: First-order logic is capable of expressing facts about some or all objects in the universe.


First-Order logic does not only use predicate and terms for making atomic sentences but also uses another way, which is equality in FOL. For this, we can use equality symbols which specify that the two terms refer to the same object.

Example: Brother (John) = Smith.

As in the above example, the object referred by the Brother (John) is similar to the object referred by Smith. The equality symbol can also be used with negation to represent that two terms are not the same objects.

Example: ¬(x=y) which is equivalent to x ≠y.

FOL inference rules for quantifier:

As propositional logic we also have inference rules in first-order logic, so following are some basic inference rules in FOL:

  • Universal Generalization
  • Universal Instantiation
  • Existential Instantiation
  • Existential introduction

1. Universal Generalization:

  • Universal generalization is a valid inference rule which states that if premise P(c) is true for any arbitrary element c in the universe of discourse, then we can have a conclusion as ∀ x P(x).
  • It can be represented as: Inference in First-Order Logic.
  • This rule can be used if we want to show that every element has a similar property.
  • In this rule, x must not appear as a free variable.

Example: Let's represent, P(c): "A byte contains 8 bits", so for ∀ x P(x) "All bytes contain 8 bits.", it will also be true.

2. Universal Instantiation:

  • Universal instantiation is also called as universal elimination or UI is a valid inference rule. It can be applied multiple times to add new sentences.
  • The new KB is logically equivalent to the previous KB.
  • As per UI, we can infer any sentence obtained by substituting a ground term for the variable.
  • The UI rule state that we can infer any sentence P(c) by substituting a ground term c (a constant within domain x) from ∀ x P(x) for any object in the universe of discourse.
  • It can be represented as:Inference in First-Order Logic.


IF "Every person like ice-cream"=> ∀x P(x) so we can infer that
"John likes ice-cream" => P(c)

Example: 2.

Let's take a famous example,

"All kings who are greedy are Evil." So let our knowledge base contains this detail as in the form of FOL:

∀x king(x) ∧ greedy (x) → Evil (x),

So from this information, we can infer any of the following statements using Universal Instantiation:

  • King(John) ∧ Greedy (John) → Evil (John),
  • King(Richard) ∧ Greedy (Richard) → Evil (Richard),
  • King(Father(John)) ∧ Greedy (Father(John)) → Evil (Father(John)),

3. Existential Instantiation:

  • Existential instantiation is also called as Existential Elimination, which is a valid inference rule in first-order logic.
  • It can be applied only once to replace the existential sentence.
  • The new KB is not logically equivalent to old KB, but it will be satisfiable if old KB was satisfiable.
  • This rule states that one can infer P(c) from the formula given in the form of ∃x P(x) for a new constant symbol c.
  • The restriction with this rule is that c used in the rule must be a new term for which P(c ) is true.
  • It can be represented as:Inference in First-Order Logic


From the given sentence: ∃x Crown(x) ∧ OnHead(x, John),

So we can infer: Crown(K) ∧ OnHead( K, John), as long as K does not appear in the knowledge base.

  • The above used K is a constant symbol, which is called Skolem constant.
  • The Existential instantiation is a special case of Skolemization process.

4. Existential introduction

  • An existential introduction is also known as an existential generalization, which is a valid inference rule in first-order logic.
  • This rule states that if there is some element c in the universe of discourse which has a property P, then we can infer that there exists something in the universe which has the property P.
  • It can be represented as: Inference in First-Order Logic
  • Example: Let's say that,
    "Priyanka got good marks in English."
    "Therefore, someone got good marks in English."

Generalized Modus Ponens Rule:

For the inference process in FOL, we have a single inference rule which is called Generalized Modus Ponens. It is lifted version of Modus ponens.

Generalized Modus Ponens can be summarized as, " P implies Q and P is asserted to be true, therefore Q must be True."

According to Modus Ponens, for atomic sentences pi, pi', q. Where there is a substitution θ such that SUBST (θ, pi',) = SUBST(θ, pi), it can be represented as:

Inference in First-Order Logic


We will use this rule for Kings are evil, so we will find some x such that x is king, and x is greedy so we can infer that x is evil.

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