Ethics, Part IV, Proposition 29

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.