  This paper presents Integrated Information Theory (IIT) 4.0. IIT aims to
account for the properties of experience in physical (operational) terms. It
identifies the essential properties of experience (axioms), infers the
necessary and sufficient properties that its substrate must satisfy
(postulates), and expresses them in mathematical terms. In principle, the
postulates can be applied to any system of units in a state to determine
whether it is conscious, to what degree, and in what way. IIT offers a
parsimonious explanation of empirical evidence, makes testable predictions, and
permits inferences and extrapolations. IIT 4.0 incorporates several
developments of the past ten years, including a more accurate translation of
axioms into postulates and mathematical expressions, the introduction of a
unique measure of intrinsic information that is consistent with the postulates,
and an explicit assessment of causal relations. By fully unfolding a system's
irreducible cause-effect power, the distinctions and relations specified by a
substrate can account for the quality of experience.
