# What is current

*Last reviewed 2026-08-09.*

The current formulation of IIT is IIT 4.0, as set out in
[Albantakis et al. (2023)](/references/albantakis-2023b), with system integrated
information (φ_s) defined as in
[Mayner, Marshall & Tononi (2026)](/references/mayner-2026), which adds the
intrinsic-information requirement.

For an account without the mathematics, read
[Tononi & Boly (2025)](/references/tononi-2025b), "Integrated Information
Theory: A Consciousness-First Approach to What Exists".

[PyPhi 2.0](https://github.com/wmayner/pyphi) computes these quantities.

## Earlier formulations

Earlier formulations of the theory include IIT 3.0
([Oizumi, Albantakis & Tononi 2014](/references/oizumi-2014)) and IIT 2.0.