Therefore, if anything were bad for us through that quality which it has in common with our nature, it would be able itself to diminish or check that which it has in common with our nature, which (III.
iv.) is absurd.

Wherefore nothing can be bad for us through that quality which it has in common with us, but, on the other hand, in so far as it is bad for us, that is (as we have just shown), in so far as it can diminish or check our power of action, it is contrary to our nature.
Q.E.D.
PROP.XXXI.

In so far as a thing is in harmony with our nature, it is necessarily good.
Proof .-- In so far as a thing is in harmony with our nature, it cannot be bad for it.

It will therefore necessarily be either good or indifferent.

If it be assumed that it be neither good nor bad, nothing will follow from its nature (IV.Def.