Universal instantiation

Rules of inference
Propositional calculus
Modus ponens Modus tollens Modus ponendo tollens Conjunction introduction Simplification Disjunction introduction Disjunction elimination Disjunctive syllogism Hypothetical syllogism Constructive dilemma Destructive dilemma Biconditional introduction Biconditional elimination
Predicate calculus
Universal generalization Universal instantiation Existential generalization Existential instantiation

In logic universal instantiation (UI, also called universal specification, and sometimes confused with Dictum de omni) is an inference from a truth about each member of a class of individuals to the truth about a particular individual of that class. It is generally given as a quantification rule for the universal quantifier but it can also be encoded in an axiom. It is one of the basic principles used in quantification theory.

Example: "All dogs are mammals. Fido is a dog. Therefore Fido is a mammal."

In symbols the rule as an axiom schema is

$\forall x \, A(x) \Rightarrow A(a/x),$

for some term a and where $A(a/x)$ is the result of substituting a for all free occurrences of x in A.

And as a rule of inference it is

from ⊢ ∀x A infer ⊢ A(a/x),

with A(a/x) the same as above.

Irving Copi noted that universal instantiation "...follows from variants of rules for 'natural deduction', which were devised independently by Gerhard Gentzen and Stanislaw Jaskowski in 1934." ^[1]

References

^ pg. 71. Symbolic Logic; 5th ed.