  Isolated quantum system in a pure state may be perceived as thermal if only
substantially small fraction of all degrees of freedom is probed. We propose
that in a chaotic quantum many-body system all states with sufficiently small
energy fluctuations are approximately thermal. We refer to this hypothesis as
Canonical Universality (CU). The CU hypothesis complements the Eigenstate
Thermalization Hypothesis (ETH) which proposes that for chaotic systems
individual energy eigenstates are thermal. Integrable and MBL systems do not
satisfy CU. We provide theoretical and numerical evidence supporting the CU
hypothesis.
