Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, so it doesn't bother you any more.
As quoted in: Johannes Karl Fink, Physical Chemistry in Depth (Springer Science & Business Media, 1992), p. 1. No primary source is given in that book.