The contemplation of the six sense bases

3. The Six Internal and External Sense Bases

And further, monks, a monk lives contemplating mental objects in the mental objects of the six internal and the six external sense-bases.

How, monks, does a monk live contemplating mental objects in the mental objects of the six internal and the six external sense-bases?

Herein, monks, a monk knows the eye and visual forms and the fetter that arises dependent on both (the eye and forms); he knows how the arising of the non-arisen fetter comes to be; he knows how the abandoning of the arisen fetter comes to be; and he knows how the non-arising in the future of the abandoned fetter comes to be.

He knows the ear and sounds... the nose and smells... the tongue and flavors... the bodyand tactual objects... the mind and mental objects, and the fetter that arises dependent on both; he knows how the arising of the non-arisen fetter comes to be; he knows how the abandoning of the arisen fetter comes to be; and he knows how the non-arising in the future of the abandoned fetter comes to be.

Thus he lives contemplating mental objects in mental objects internally, or he lives contemplating mental objects in mental objects externally, or he lives contemplating mental objects in mental objects internally and externally. He lives contemplating origination factors in mental objects, or he lives contemplating dissolution factors in mental objects, or he lives contemplating origination-and-dissolution factors in mental objects. Or his mindfulness is established with the thought, "Mental objects exist," to the extent necessary just for knowledge and mindfulness, and he lives detached, and clings to nothing in the world. Thus, monks, a monk lives contemplating mental objects in the mental objects of the six internal and the six external sense-bases.

From the Satipatthana Sutta, translated by Nyanasatta Thera 

http://www.accesstoinsight.org/tipitaka/mn/mn.010.nysa.html