Build the idea in three steps
- A negation such as f ≠ 0 means evidence for f = 0 would yield False, a contradiction.
- Both statements must concern exactly the same object in the same space. Two different forms can behave differently.
- Applying the nonzero evidence to the zero evidence closes the impossible branch, ruling out its original assumption.
A small example
One f cannot satisfy both f ≠ 0 and f = 0. A nonzero f and a different zero g would not be a contradiction.