I, Hume defends abstruse and abstract philosophy in the face of everyone's preference for the easy and the obvious. He argues that the philosopher's job is more like the anatomist than the painter. Hume thinks that, on analogy with Newton's success in the natural world, a proper study of human nature will reveal its basic principals.
Impressions are more forceful and lively, while ideas are weaker.
By and large, impressions are immediate sensations while ideas are the memories of sensation, or the anticipation of a sensation. Hume argues that impressions are the originals, while ideas are the derived copies. The missing shade of blue provides an exception which Hume finds surprisingly untroubling. With his account of impressions and ideas, and of their associative mechanisms, Hume is ready to give his account of the workings of human understanding or reasoning.