P29: Any singular thing whose nature is entirely different from ours can neither aid nor restrain our power of acting, and absolutely, no thing can be either good or evil for us, unless it has something in common with us.

Dem.: The power of each singular thing, and consequently (by IIP10 C), man’s power, by which he exists and produces an effect, is not determined except by another singular thing (by IP28), whose nature must be understood (by IIP6) through the same attribute through which human nature is conceived. Our power of acting, therefore, however it is conceived, can be determined, and hence aided or restrained, by the power of another singular thing which has something in common with us, and not by the power of a thing whose nature is completely different from ours.

And because we call good or evil what is the cause of Joy or Sadness (by P8), i.e. (by IIIP11 S), what increases or diminishes, aids or restrains, our power of acting, a thing whose nature is completely different from ours can be neither good nor evil for us, q.e.d.