Ousia expresses being-in-itself.
The sophists presuppose Heraclitus’ “permanent flux.”
The worlds of Knowledge (populated by Forms, Ideas) and Opinion (Sense-perception) are fully separated. The former is the world of Being, and the latter is Becoming. Hence, the latter can be seen as a placement of Heraclitus.
Any sense-perception can only lead to opinion; knowledge is a faculty of the soul which finds its object in the level of being over and against the world of opinion which is known as the sensible world.
Lovers of sight vs. lovers of knowledge