I vaguely remember from an Analysis lecture when Mathematical rigor wasn't such a thing, the professor mentioned a famous Mathematician (maybe Euler?) who had a

I vaguely remember from an Analysis lecture when Mathematical rigor wasn't such a thing, the professor mentioned a famous Mathematician (maybe Euler?) who had a notebook with hundreds of proofs where many of the visual ones didn't turn out right. Definitely nice for intuition though since this is oftentimes lacking