\begin{center}
\vspace{-0.5cm}
\begin{prooftree}
\AxiomC{$ \varphi $}
\AxiomC{$ \psi $}
\RightLabel{$ \land $i}
\BinaryInfC{$ \varphi \land \psi $}
\end{prooftree}
\end{center}

If we have two formulas that are known to be true separately, then we can
conclude that the conjunction of the two premises must also be true.
