Assume we have a Gödel numbering of formulas $\#\varphi$. A *truth definition* for a model $\mathcal{M}$ (set or class, satisfying enough set theory) is a formula $T(x)$ such that $\mathcal{M}\vDash T(\#\varphi)\leftrightarrow\varphi$ for any sentence $\varphi$. **Therom (Tarski).** A truth definition doesn't exist for any model $\mathcal{M}$.