---
tag: DPMKT2KRB8
kind: concept
name: Independence-friendly logic
aliases: [if-logic, team-semantics]
sources:
  - wikipedia: en:1974007@r1305487255
    locator: Independence-friendly logic
    note: adopted
    mention: wikipedia://en/1974007
    sections: [Summary]
same_as:
  wikidata: Q17101558
  wikipedia: Independence-friendly logic
inventory: [9VWHSCKPFF]
---

## Summary

**Independence-friendly logic** (**IF logic**; proposed by Jaakko Hintikka and Gabriel Sandu (philosopher) in 1989) is an extension of classical [[first-order logic|086QKPF0A6]] (FOL) by means of slashed quantifiers of the form $(\exists v/V)$ and $(\forall v/V)$, where $V$ is a finite set of variables. The intended reading of $(\exists v/V)$ is "there is a $v$ which is functionally independent from the variables in $V$". IF logic allows one to express more general patterns of dependence between variables than those which are implicit in first-order logic. This greater level of generality leads to an actual increase in expressive power; the set of IF sentences can characterize the same classes of structures as existential second-order logic ($\Sigma^1_1$).