Say, " $\psi \Rightarrow \varphi$ " is a theorem and $\psi$ is essential in the hypothesis.
I don't understand what's the meaning of essential.
Here's what i guess;
If $[\psi \Rightarrow \Phi] \bigwedge \neg [\Phi \Rightarrow \psi] \Rightarrow \neg [\Phi \Rightarrow \varphi]$, then we call $\psi$ is essential in the hypothesis.
Am i correct?