Full Idea
The third of the Galilean doctrines of science is that the best, the truly scientific theories, describe the 'essences' or the 'essential natures' of things - the realities which lie behind the appearances. They are ultimate explanations.
Gist of Idea
Galilean science aimed at true essences, as the ultimate explanations
Source
Karl Popper (Conjectures and Refutations [1963], 3.3)
Book Reference
Popper,Karl: 'Conjectures and Refutations' [RKP 1965], p.104
A Reaction
This seems to be the seventeenth century doctrine which was undermined by Humeanism, and hence despised by Popper, but is now making a comeback, with a new account of essence and necessity.