>[!info] Definition >$\begin{gathered} \mathcal{M}\vDash Ixy\varphi\left(x,\bar{b}\right)\psi\left(y,\bar{b}\right)\iff\\ \left|\left\{ a\in M\mid\mathcal{M}\vDash\varphi\left(a,\bar{b}\right)\right\} \right|=\left|\left\{ a\in M\mid\mathcal{M}\vDash\psi\left(a,\bar{b}\right)\right\} \right| \end{gathered}$