Proof.- The power of every individual thing, and consequently the power of man, whereby he exists and operates, can only be determined by an individual thing (I:xxviii.), whose nature (II:vi.) must be understood through the same nature as that, through which human nature is conceived. Therefore our power of activity, however it be conceived, can be determined and consequently helped or hindered by the power of any other individual thing, which has something in common with us, but not by the power of anything, of which the nature is entirely different from our own; and since we call good or evil that which is the cause of pleasure or pain (IV:viii.), that is (III:xi.Note), which increases or diminishes, helps or hinders, our power of activity; therefore, that which is entirely, different from our nature can neither be to us good nor bad. Q.E.D. Prop. XXX. A thing cannot be bad for us through the quality which it has in common with our nature, but it is bad for us in so far as it is contrary to our nature. Proof.- We call a thing bad when it is the cause of pain (IV:viii.), that is (by the Def., which see in III:xi.Note), when it diminishes or checks our power of action. 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.i.), which tends to the preservation of our nature, that is (by the hypothesis), which tends to the preservation of the thing itself; but this (III:vi.) is absurd; therefore, in so far as a thing is in harmony with our nature, it is necessarily good. Q.E.D.