Excerpt from the chapter on "A Comprehensive Practice" in Lessons in Practical Buddhism
The first step in our progress, the first factor of enlightenment, is called "sati". "sati" is a word that should be familiar to most Buddhists; unfortunately, however, it is often understood quite loosely, even incorrectly. Generally translated as "mindfulness", it is usually taken to mean "awareness" or "alertness", both of which are ostensibly positive qualities of mind. "sati", however, means neither.
The word "sati" comes from a root (sara) that means to remember, or recollect. This root is used in the standard form of "going for refuge" to the Buddha, his teachings and his enlightened followers, for example: "buddhaṃ saraṇaṃ gacchāmi" - "I go to the Buddha as a saraṇa". The word "saraṇa" is generally translated as "refuge", which it can indeed mean. The word also means, however, "object of recollection", i.e. something to recollect in times of difficulty.
Indeed, this is exactly what the Buddha encouraged us to do when we are in distress. He said, "maṃ anussareyyātha" - "you should recollect me", because thoughts on the perfection of the Buddha would console us in times of despair. Similar practices exist in many of the world's religions to bring faith and courage in times of difficulty, there is nothing particularly Buddhist in it.
From this explanation, though, we can see that the word "sati", the very basis on which we are to build our practice, has something to do with calling to mind, or keeping in mind. The word sati is sometimes used to refer to recollecting about things that have happened in the past or future as well. In the context of the factors of enlightenment, however, it refers only to recollection of the present moment. What it really means is to call to mind the objective nature of the experience, eschewing all projection, extrapolation or judgement about the object.
According to the Abhidhamma, sati arises based on fortified recognition (thīra-saññā). Whereas ordinary recognition (saññā) is not enough to keep the mind in objective awareness, once we fortify or reaffirm this recognition, not letting the mind move beyond simple awareness of the object for what it is, our minds will penetrate the nature of the object to the core, dispelling all doubt as its essential nature as something worth clinging to or not.
So, sati would be better translated as "recognition", and this is how it has been referred to throughout this chapter. It is deliberate and sustained recognition that in turn allows us to see the objects of experience as they truly are.
This explanation, which may seem a bit dry to some readers, is necessary to help us understand what the Buddha really meant in the Satipaṭṭḥāna Sutta, when he said, as quoted earlier, "when walking, one fully comprehends: 'I am walking'." It is clear that he did not mean that we should be aware that we are walking, since awareness is common to animals and ordinary people alike. Simply recognizing that we are walking is something that requires no meditative training whatsoever.
To "fully comprehend" (pajānāti), one must cultivate the mental quality of "sati" or fortified recognition (thīra-saññā) by reminding oneself of the essential nature of the experience, as in "walking". Reminding oneself of what one already recognizes in this way is equivalent to arresting the mind's natural progression into projecting, judging, clinging, seeking, building up, and finally suffering.
Another way of understanding this activity of fortifying one's recognition is as a mantra, a traditional meditative tool that has been used for millennia by meditators both Buddhist and non. A mantra is used to focus the mind on an object, arresting the mind's natural inclination to jump from object to object. It is traditionally used to focus on a conceptual object, something a meditator conjures up in the mind, a picture or a spiritual object like a god or angel.
A mantra can, however, be used in much the same way in order to fix the mind on a real object as well, be it a physical sensation, a feeling, a thought, or an emotion. This is one way of understanding the word "sati" in the context of the Satipaṭṭhāna Sutta; it is the use of a mantra to stabilize and fortify one's bare recognition of an experience for what it is, allowing one to see clearly without prejudice or projection and thus remove any misapprehensions based on delusion or ignorance.
Once we cultivate sati, our minds will naturally incline towards observing the nature of phenomena; just as how a person who sees a tiger also sees its stripes, observation of the characteristics of every object of one's experience will become unavoidable. One will be forced to see clearly the true nature of everything one clings to, as well as the result of such clinging. One will see that the objects of experience are universally impermanent, unsatisfying, and uncontrollable; one will see that clinging to such entities is akin to banging one's head off of a wall - painful and utterly without purpose.