Zeno and the Stoics - The Criterion of Truth

The sceptics of the academy often seem to have had little more on their minds than undermining the stoics. Since they professed no doctrines of their own, what they brought to the table was not a rival theory like that of the epicureans but an arsenal of dialectical weaponry designed to undercut and cast doubt upon the stoic position. They were willing to fight on any ground, from logic to physics to ethics. But at the core of the dispute, inevitably, was the nature of knowledge itself. For the stoics, the criterion of truth is what they call a cognitive impression, that is, an impression about how things are that corresponds to how things are and that cannot be misleading