The following notes are a transcription and a translation into English of lecture notes (192 pages!) of de Bruijn's course on combinatorics, originally given in the 70s and 80s:http://goo.gl/ZXO5AV
you may be surprised where they are hosted. From the introduction:Combinatorics is mainly about counting. The nice thing about counting is that you learn something about the thing that you are counting. While counting, you notice that your knowledge about the subject is not sufficiently precise, and so counting becomes an educational activity. When two sets have the same number of elements, you try to understand why that is the case by establishing some natural bijection between the two.N. G. de Bruijn recalls that sometime around 1975, he counted the number of a certain type of logics with three variables. There were a lot of them; some number with 14 digits. A little while later he saw an article that was about something completely different, but it concluded that the number of those objects was exactly that same number with 14 digits. He got curious and read the article very carefully. Indeed, if you thought deeply about it you could see that you could interpret those objects also as logics.
(via darij grinberg on MathOverflow)