  One of the basic observations of the classical world is that physical
entities are real and can be distinguished from each other. However, within
quantum theory, the idea of physical realism is not well established. A
framework to analyse how observations in experiments can be described using
some physical states of reality was recently developed, known as ontological
models framework. Different principles when imposed on the ontological level
give rise to different theories, the validity of which can be tested based on
the statistics generated by these theories. Using the ontological models
framework, we formulate a novel notion of classicality termed
ontic-distinguishability, which is based upon the physical principles that in
classical theories extremal states are physical states of reality and every
sharp measurement observes the state of the system perfectly. We construct a
communication task in which the success probability is bounded from above for
ontological models satisfying the notion of ontic-distinguishability. Contrary
to previous notions of classicality which either required systems of dimension
strictly greater than two or atleast three preparations, a violation of
ontic-distinguishability can be observed using just a pair of qubits and a pair
of incompatible measurements. We further show that violation of previously
known notions of classicality such as preparation non-contextuality and Bell's
local causality is a violation of ontic-distinguishability.
