One should know the characteristics of space. All those who do not know emptiness claim non-existence is existence. The perverted regard a mirage as water. Ignorance about the truth is the cause of samsara. Cittamatrins and heretics like the Sankhyas and the rest maintain that upaya and prajna are separate. Each has his own theory. This is the same as maintaining that a dead tree has flowers.

-- Marpa