You have to distinguish between "understanding" and "reasoning". Understanding means "understanding what ... does", where as reasoning (in "functional" jargon)

You have to distinguish between "understanding" and "reasoning". Understanding means "understanding what ... does", where as reasoning (in "functional" jargon) means (more like) "proving what ... does". While it's quite easy to understand what e.g. `c = a + b;` in C does, reasoning about it is comparably hard, as there can be UB (overflow of a signed integer type) involved. In the extreme, when you have an actual proof (in a dependently typed language), you only have to understand it, but no need to reason about it as the proof is already there ;) [insert some dependently typed vector example here :D]