[These notes appear to refer to Ms. 2857 in the Besançon archives, “De la Pratique des révolutions,” a draft for a work abandoned in favor of The General Idea of the Revolution in the 19th Century. This is, of course, just one of the places where Proudhon uses “Destruam et ædificabo” as a kind of motto, but it is interesting to find it here, close to the “watershed” between Proudhon’s “critical” and “constructive” projects.]
P.-J. Proudhon, Carnets, Vol. 4 (Carnet no. 9, 27-28; March 1851): 215.
Revolutionary practice. — Book I, chapter I. — If I start with cannibalism, it is not as a vain show of horrors. It is necessary to show that great truth that is everything in man, Progress. The state of nature is the bestial state, from which we have first drawn fetishist, polytheist religion; the path that we have made, we have made with reason and liberty.
Crude, savage reason at first, ferocious and jealous, which sanctified and purged itself every day. 
It is a new religion that we will establish, a social, human religion, which we must draw out more and more.
Rousseau gave its prelude in the Emile, attempting in turn to make it metaphysics, dogma and morals: a task beyond his powers.
We begin anew, with more powerful materials and a new insight.
We have achieved a methodical critique, destruam; we begin the edifice of the new faith: aedificabo.
Revolutionary practice should be nothing but that. — Do not forget it.
[Working translation by Shawn P. Wilbur]