The Pali Sutta on the Applications of Mindfulness (Satipatthana, M. i. 55) is certainly a very important Sutta which has been condensed under four general headings of body, feelings, mind, and mental objects. However, this Sutta’s most difficult problem lies in what mindfulness or sati, itself, exactly means. This is when dictionaries start to fail.
The question of sati (in Sanskrit smriti) itself, is very important for the reason that it is what gets us through this particular Sutta. We read that with mindfulness one "fares along independently of and not grasping anything in the world" (M. i. 62). Fully understanding the meaning of sati, in other words, tends towards the hyper-subtle—it is out of this world.
First of all, we can say of sati that it is impersonal with regard to the four presentations of body, feeling, mind, and mental objects which come in front or before it (to be mindful of anything assumes a kind of spiritual detachment—not spatial). However, while ordinary mindfulness is aware of the general—already here—presentation of body, feelings, mind and mental objects, the Buddha's sati appears to be directed more subtly, to be before presentation. In the example of respiration, mindfulness is before respiration, both the in and out breath. It doesn’t follow respiration, in other words—it is always before or prior to it.
Such mindfulness works to disentangle and distingusih the self from common secondary presentations, all of which, come under the heading of the Five Aggregates (khandhas/skandhas).