Full Idea
Whenever we find some quality in a subject, we ought to believe that if we understood the nature of both the subject and the quality we would conceive how the quality could arise from it.
Gist of Idea
Qualities should be predictable from the nature of the subject
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], Pref 66)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.66
A Reaction
This is the idea that powers are prior to properties, which seems right to me. I take essence to be something like the best explanation of qualities.