P30: No thing can be evil through what it has in common with our nature; but insofar as it is evil for us, it is contrary to us.

Dem.: We call evil what is the cause of Sadness (by P8), i.e. (by the Definition of Sadness, see IIIP11 S), what diminishes or restrains our power of acting. So if a thing were evil for us through what it has in common with us, then the thing could diminish or restrain what it has in common with us. But (by IIIP4) this is absurd. Therefore, no thing can be evil for us through what it has in common with us. On the contrary, insofar as it is evil, i.e. (as we have already shown), insofar as it can diminish or restrain our power of acting, it is contrary to us (by IIIP5), q.e.d.