This may sound silly.
I used to remember studying this in physics class and I thought of asking it in physics.stackexchange and then later I decided to ask it here itself.
Let's say, under some conditions,a variable $x$ can either have $2$ values such as $y$ or $z$.
Assume, we need to prove that $x = y$, then sometimes, they used to prove that $x \neq z$.
Since $x$ has $2$ possible values and we proved $x\neq z$, so obviously, we proved that $x = y$ indirectly.
These kind of proving mechanism, I used to remember it in many places.
What is this theorem called ? There is a common term is there for this, like "theory of blah, blah,..." (Hoping not to get a down vote for this dumb question)