  Quantum mechanics is usually formulated with an implicit assumption that
agents who can observe and interact with the world are external to it and have
a classical memory. However, there is no accepted way to define the
quantum-classical cut and no a priori reason to rule out fully quantum agents
with coherent quantum memories. In this work, we introduce an entirely quantum
notion of measurement, called a sensation, to account for quantum agents that
experience the world through quantum sensors. Sensations eschew probabilities
and instead describe a deterministic flow of quantum information. We quantify
the information gain and disturbance of a sensation using concepts from quantum
information theory and find that sensations always disturb at least as much as
they inform. Viewing measurements as sensations could lead to a new
understanding of quantum theory in general and to new results in the context of
quantum networks.
