Joseph-Louis Lagrange (1736 - 1813)

[a quotation from De Morgan]
Lagrange, in one of the later years of his life, imagined that he had overcome the difficulty (of the parallel axiom). he went so far as to write a paper, which he took with him to the Institute, and began to read it. But in the first paragraph something struck him which he had not observed: he muttered: "Il faut que j'y songe encore ", and put the paper in his pocket.
[I must think about it again.]

A De Morgan Budget of Paradoxes