P31: Insofar as a thing agrees with our nature, it is necessarily good.

Dem.: Insofar as a thing agrees with our nature, it cannot be evil (by P30). So it must either be good or indifferent. If the latter is posited, viz. that it is neither good nor evil, then (by A3) nothing will follow from its nature that aids the preservation of our nature, i.e. (by hypothesis), that aids the preservation of the nature of the thing itself. But this is absurd (by IIIP6). Hence, insofar as it agrees with our nature, it must be good, q.e.d.

Cor.: From this it follows that the more a thing agrees with our nature, the more useful, or better, it is for us, and conversely, the more a thing is useful to us, the more it agrees with our nature.

For insofar as it does not agree with our nature, it will necessarily be different from it or contrary to it. If it is different from it, then (by P29) it can be neither good nor evil. And if it is contrary, then it will also be contrary to that which agrees with our nature, i.e. (by P31), contrary to the good, or evil. Nothing, therefore, can be good except insofar as it agrees with our nature. So the more a thing agrees with our nature, the more useful it is, and conversely, q.e.d.