While agreeing with the Sautrantikas regarding the reality of both the mental and the non-mental, the Vaibhasikas, like many modern neo-realists, point out that unless we admit that external objects are perceived by us, their existence cannot be known in any other way.’
Inference of fire from the perception of smoke is possible, because in the past we have perceived both smoke and fire together. One who has never perceived fire previously cannot infer its existence from the perception of smoke.
If external objects were never perceived, as the Sautrantikas hold, then they could not even be inferred, simply from their mental forms.
To one unacquainted with an external object, the mental form would not appear to be the copy or the sign of the existence of an extra-mental object, but as an original thing which does not owe its existence to anything outside the mind.
Either, therefore, we have to accept subjective idealism (vijnana-vada) or, if that has been found unsatisfactory, we must admit that the external object is directly known. The Vaibhasikas thus come to hold ‘a theory of direct realism’ (bahya, pratyaksa-vada).
The Abhidhamma treatises formed the general foundation of the philosophy of the realists. The Vaibhasikas followed exclusively a particular commentary, Vibhasa (or Abhidhamm or mahavibhasa) on an Abhidhamma treatise (Abhidharmma-jnana- prastnana) Hence their name.