Full Idea
Anything which occurs in what is strictly a substance must be a case of 'action' in the metaphysically rigorous sense of something which occurs in the substance spontaneously, arising out of its own depths.
Gist of Idea
All occurrence in the depth of a substance is spontaneous 'action'
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.21)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.210
A Reaction
I love this idea, which fits in with scientific essentialism. The question is whether Leibniz has idenified the end point of all explanations. Cutting edge physics is trying to give further explanations for what seemed basic, such as mass and gravity.
Related Ideas
Idea 12783 Primitive force is what gives a composite its reality [Leibniz]
Idea 13169 I call Aristotle's entelechies 'primitive forces', which originate activity [Leibniz]