the 4 'foundations of mindfulness', lit. 'awareness of mindfulness' (sati-upatthāna), are:
For sati, s. prec.
A detailed treatment of this subject, so important for the practice of Buddhist mental culture, is given in the 2 Satipatthāna Suttas (D.22; M.10), which at the start as well as the conclusion, proclaim the weighty words: "The only way that leads to the attainment of purity, to the overcoming of sorrow and lamentation, to the end of pain and grief, to the entering of the right path, and to the realization of Nibbāna is the 4 foundations of mindfulness."
After these introductory words, and upon the question which these 4 are, it is said that the monk dwells in contemplation of the body, the feelings, the mind, and the mind-objects, "ardent, clearly conscious and mindful, after putting away worldly greed and grief."
These 4 contemplations are in reality not to be taken as merely separate exercises, but on the contrary, at least in many cases, especially in the absorptions, as things inseparably associated with each other. Thereby the Satipathāna Sutta forms an illustration of the way in which these 4 contemplations relating to the 5 groups of existence (khandha) simultaneously come to be realized, and finally lead to insight into the impersonality of all existence.
(1) The contemplation of the body (kāyanupassanā) consists of the following exercises:
(2) All feelings (vedanānupassanā) that arise in the meditator he clearly perceives, namely:
(3) He further clearly perceives and understands any state of consciousness or mind (cittānupassanā), whether it is
(4) Concerning the mind-objects (dhammānupassanā),
The 4 contemplations comprise several exercises, but the Satipatthāna should not therefore be thought of as a mere collection of meditation subjects, any one of which may be taken out and practised alone. Though most of the exercises appear also elsewhere in the Buddhist scriptures, in the context of this sutta they are chiefly intended for the cultivation of mindfulness and insight, as indicated by the repetitive passage concluding each section of the sutta (see below).
The 4 contemplations cover all the 5 groups of existence (khandha), because mindfulness is meant to encompass the whole personality. Hence, for the full development of mindfulness, the practice should extend to all 4 types of contemplation, though not every single exercise mentioned under these four headings need be taken up. A methodical practice of Satipatthāna has to start with one of the exercises out of the group 'contemplation of the body', which will serve as the primary and regular subject of meditation: The other exercises of the group and the other contemplations are to be cultivated when occasion for them arises during meditation and in everyday life.
After each contemplation it is shown how it finally leads to insight-knowledge: "Thus with regard to his own body he contemplates the body, with regard to the bodies of others he contemplates the body, with regard to both he contemplates the body. He beholds how the body arises and how it passes away, beholds the arising and passing away of the body. 'A body is there' (but no living being, no individual, no woman, no man, no self, nothing that belongs to a self; neither a person, nor anything belonging to a person; Com.): thus he has established his attentiveness as far as it serves his knowledge and mindfulness, and he lives independent, unattached to anything in the world.''
In the same way he contemplates feeling, mind and mind-objects.
In M.118 it is shown how these four foundations of mindfulness may be brought about by the exercise of mindfulness on in-and-out breathing (ānāpāna-sati).