  Quantum theory has an epistemic horizon, i.e. exact values cannot be assigned
simultaneously to incompatible physical quantities. As shown by Spekkens' toy
theory, positing an epistemic horizon akin to Heisenberg's uncertainty
principle in a classical mechanical setting also leads to a plethora of quantum
phenomena. We introduce a deterministic theory - nomic toy theory - in which
information gathering agents are explicitly modelled as physical systems. Our
main result shows the presence of an epistemic horizon for such agents. They
can only simultaneously learn the values of observables whose Poisson bracket
vanishes. Therefore, nomic toy theory has incompatible measurements and the
complete state of a physical system cannot be known. The best description of a
system by an agent is via an epistemic state of Spekkens' toy theory. Our
result reconciles us to measurement uncertainty as an aspect of the
inseparability of subjects and objects. Significantly, the claims follow even
though nomic toy theory is essentially classical. This work invites further
investigations of epistemic horizons, such as the one of (full) quantum theory.
