The Four Applications of Mindfulness are part of the Thirty-seven Wings of Enlightenment (i.e., Seven Wings of Bodhi, weightfold path, Five Faculties, Five Powers, Four Bases of Psychic Power, Four Applications of Mindfulness, Four Types of Upright Diligence) that comprise the traditional description of Path, the last of the four holy truths.

“The Four Applications of Mindfulness were given by the Buddha as a dwelling place for the Bhikshus after his departure into nirvana.” (DFS IV 760)

Contemplation of the Body as Impure
“Our bodies are unclean things.…Perspiration flows from the entire body, and once you perspire, you smell. Tears and matter flow from the eyes. Wax oozes from the ears and mucus flows from the nose. Saliva and phlegm flow from the mouth. These seven orifices are always leaking unclean substances. Then add the eliminatory orifices and you have nine holes which constantly ooze with impurities. Everyone is familiar with these impurities. In our flesh and blood there are many kinds of bacteria as well which are impure. Someone may not believe this at all, but in the future advances in science willwithout a doubt prove that the flesh and blood are unclean. It’s all very complex, especially when people eat a lot of strange things which get into their systems and do strange things. The matter in the digestive system is also unclean. Therefore, why should you be so caught up in working for your body? First of all, contemplate the body as impure.

Contemplation of Feeling as Suffering
“Secondly, contemplate feelings as suffering. Pleasurable sensations are enjoyable at first, but one soon grows tired of them, and they become disagreeable. It’s a very obvious principle that there is nothing much to pleasure in itself.

Contemplation of Thoughts as Impermanent
“Thirdly, contemplate thoughts as impermanent. Thought after thought changes and moves on. Thoughts are like waves on the sea. When one thought passes, another takes its place. Coming into being and ceasing to be, coming into being and ceasing to be, thoughts do not stop.…Past, present, and future—none of the three phases of thought can be got at. Therefore, contemplate thought as impermanent. We are never aware of where our thoughts have gone off to. Mencius said, ‘If people’s chickens and dogs run off, they go after them. But if their thoughts run off, they don’t know to go after them.…’ When you have polluted thinking, that is just your mind running off.…

Contemplation of Dharmas as Devoid of Self
“Not only is there no self, there are no dharmas either! Make empty both people and dharmas. Empty emptiness as well.” (DFS IV 608-610)

四念處