Now how does sati differ from samādhi? The analogy is given of tying up a wild animal, freshly caught from the jungle. The animal runs about wildly. Without the rope to hold it, it would surely escape. So we must tie the animal to a stake, so that instead of escaping, the animal can only run around the vicinity of the stake. The rope is comparable to sati. Sati is that which pulls the mind back, or pulls a particular sensation to the mind, or pulls the mind to a particular sensation. It may also be said to hold the mind to, or force it to stay with, a particular sensation. That which holds or forces the mind, preventing it from wandering too far, is sati.
Now if we restrain the mind until it calms down and stays still of its own accord, this is samādhi. Like the wild animal, once it is tied it can't roam about because it is held by the rope, which we compared to sati. After a time the animal tires and lies down calmly. The animal lying calmly is like the mind which has samādhi, which is firm and still.
Thus sati and samādhi are closely related. Sati is a factor which helps to develop samādhi.
Now in addition to aiding in the development of samādhi, sati is also a factor which can influence the mind to develop wisdom. All things that we can think of or reflect on are called sensations (ārammana). If all sensations disappeared we would no longer have anything to reflect on, because there would be nothing there. In order for a sensation to stay with us there must be something to hold it down. Sati is what holds the theme of contemplation to our attention, so that wisdom can consider it and develop understanding. Thus paññā (wisdom), too, cannot function without sati. There must be sati to hold things to consciousness so that we can see them and reflect on them. We must have, before anything else, sati.