Full Idea
It can be said that there is nothing in the understanding which does not come from the senses, except the understanding itself, or that which understands.
Gist of Idea
There is nothing in the understanding but experiences, plus the understanding itself, and the understander
Source
Gottfried Leibniz (Letters to Queen Charlotte [1702], 1702)
Book Reference
Leibniz,Gottfried: 'Leibniz Selections', ed/tr. Wiener,Philip P. [Scribners 1951], p.359
A Reaction
Given that Leibniz is labelled as a 'rationalist', this is awfully close to empiricism. Not Locke's 'tabula rasa' perhaps, but Hume's experiences plus associations. Leibniz has a much loftier notion of understanding and reason than Hume does.