n.1. Roughly, its essential nature.

n.2. Both words mean ‘striving’, and both are sometimes treated as if they were English words (especially ‘conatus’). The concept was important in both Hobbes and Spinoza, for whom it was a microscopic motion, or beginning of motion, which, if not counteracted, would set off a macroscopic motion. It was a major source for Leibniz’s concept of force or energy, which he believed had to be represented by a microscopic motion of parts as long as it was not released.

n.3. The point is that, if motion is really different from rest, a moving body must be different at any instant from a stationary one. A modern analogy would be to say that a photograph of a moving object must always be slightly blurred, however short the exposure, whereas a photograph of a stationary object can be perfectly sharp. Descartes, on the other hand, didn’t believe in the continuity of motion, but held instead that the world is perpetually recreated slightly differently from one instant to the next (rather like the discrete frames on a film).