디가 니카야 2:328
SUJATOIti ajjhattaṁ vā dhammesu dhammānupassī viharati, bahiddhā vā dhammesu dhammānupassī viharati, ajjhattabahiddhā vā dhammesu dhammānupassī viharati.
And so they meditate observing an aspect of principles internally, externally, and both internally and externally.