Hello, I have a problem with this statement:

Let $\displaystyle J\ne \{0\}$ - two-sided ideal in Von Neumann algebra, then $\displaystyle J^{pr}\ne \{0\}$.

I know, that $\displaystyle J$ is two-sided ideal in Von Neumann algebra $\displaystyle <=> \quad x\in J \Rightarrow x*\in J$. What next?