Key Arguments

• What is in harmony with our nature cannot be bad and cannot be indifferent without implying nothing follows from its nature tending to preserve itself, which is absurd; thus it is necessarily good.
• Corollary: The more a thing is in harmony with our nature, the more useful or better; conversely, more useful implies more harmony, since what is different is neither good nor bad, and what is contrary is bad.

Source Quotes

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.

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. Corollary.—Hence it follows, that, in proportion as a thing is in harmony with our nature, so is it more useful or better for us, and vice versâ, in proportion as a thing is more useful for us, so is it more in harmony with our nature. For, in so far as it is not in harmony with our nature, it will necessarily be different therefrom or contrary thereto.

Key Concepts

• PROP. XXXI. In so far as a thing is in harmony with our nature, it is necessarily good.
• Corollary.—Hence it follows, that, in proportion as a thing is in harmony with our nature, so is it more useful or better for us, and vice versâ, in proportion as a thing is more useful for us, so is it more in harmony with our nature.

Context

Ethics, Part IV, Proposition XXXI with Proof and Corollary (lines 3265–3395)