Buescher Origins of Doctrine of the Two Truths (PDF)

The Complete Title is Echoes from an Empty Sky. The Origins of the Buddhist doctrine of the Two Truths.

Buddhists long ago developed a doctrine of two truths—conventional truth and ultimate truth. They have understood this doctrine in two ways. The first—which appears to have been its original meaning—referred to the sorts of true statements that the Buddha made: his “worldly,” sometimes ambiguous, conventional discourse, and his statements or discourse that plainly referred to the ultimate truth. The second way referred to the sorts of objects in the world.

This early development of the doctrine of two truths is traced here. Briefly. Each of the many early schools of Buddhism used the doctrine in internecine debates to distinguish what it saw as the Buddha’s highest teachings from what it believed other schools had mistaken as ultimate truths, or had invented altogether

The second way that Buddhists have understood the doctrine of two truths was as categories of all the objects in the universe or as the modes of objective reality.