Full Idea
Primitive forces can be nothing but the internal strivings [tendentia] of simple substances, striving by means of which they pass from perception to perception in accordance with a certain law of their nature.
Gist of Idea
Primitive forces are internal strivings of substances, acting according to their internal laws
Source
Gottfried Leibniz (Letters to Burcher De Volder [1706], 1704 or 1705)
Book Reference
Leibniz,Gottfried: 'Philosophical Essays', ed/tr. Arlew,R /Garber,D [Hackett 1989], p.181
A Reaction
'Perception' sounds a bit crazy, but he usually qualifies that sort of remark by saying that it is an 'analogy' with conscious willing souls. The 'internal strivings of substances' is a nice phrase for the basic powers in nature where explanations stop.