To get all
philosophical for a moment,
nisus is a fairly central component of
Aristotle's theory of nature. If you remember, he thought (and convinced most of our
intelligent ancestors until the
Renaissance )that the world consisted of
objects that were striving to find their natural place of
rest, or where they most
wanted to be.
So nisus is this force driving changes in nature that we observe. Ie a leaf falls because it wants to touch the ground, you visit your lover because you want to, yearn to, and won't be at rest until you do, the same is true of the sun making it's way into the heavens, the water falling in the form of droplets, each and everything concievable yearns to go to it's natural place.
It's such an intuitive, all pervading philosophy that it's hard to find fault with it.
Nevertheless Newton and his laws swept all this away, as did the realization that bodies in the universe, especially inanimate ones follow simple universal laws of motion, and don't deviate from their paths, simply 'because they want to'.