Sans les mathématiques on ne pénètre point au fond de la philosophie. Sans la philosophie on ne pénètre point au fond des mathématiques. Sans les deux on ne pénètre au fond de rien.
The above quotation, an early occurrence of which is found in this 1834 edition of Bordas-Demoulin's Le Cartésianisme, is attributed to Leibniz by many writers, e.g. Gregory Chaitin, C.-A. Laisant and (in German) Peter Ruben. It is, however, never stated exactly where this passage should be found in Leibniz's oeuvre.
My question then: is the attribution more than apocryphal/anecdotal? Did Leibniz actually write this? If so, in what language, and where is the original to be found?