I was reading a bit farther in Charles Coulson Gillispie’s Pierre-Simon Laplace, 1749 – 1827, A Life In Exact Science and reached this paragraph, too good not to share:
Wishing to study [ Méchanique céleste ] in advance, [ Jean-Baptiste ] Biot offered to read proof. When he returned the sheets, he would often ask Laplace to explain some of the many steps that had been skipped over with the famous phrase, “it is easy to see”. Sometimes, Biot said, Laplace himself would not remember how he had worked something out and would have difficulty reconstructing it.
So, it’s not just you and your instructors.
(Gillispie wrote the book along with Robert Fox and Ivor Grattan-Guinness.)