With regard to good and evil, these terms indicate nothing positive in
things considered in themselves, nor are they anything else than modes
of thought, or notions which we form from the comparison of one thing
with another. For one and the same thing may at the same time be both
good and evil or indifferent. Music, for example, is good to a
melancholy person, bad to one mourning, while to a deaf man it is
neither good nor bad. But although things are so, we must retain these
words. For since we desire to form for ourselves an idea of man upon
which we may look as a model of human nature, it will be of service to
us to retain these expressions in the sense I have mentioned.

By _good_, therefore, I understand in the following pages everything
which we are certain is a means by which we may approach nearer and
nearer to the model of human nature we set before us. By _evil_, on the
contrary, I understand everything which we are certain hinders us from
reaching that model. Again, I shall call men more or less perfect or
imperfect in so far as they approach more or less nearly to this same
model. For it is to be carefully observed, that when I say that an
individual passes from a less to a greater perfection and _vice versa_,
I do not understand that from one essence or form he is changed into
another (for a horse, for instance, would be as much destroyed if it
were changed into a man as if it were changed into an insect), but
rather we conceive that his power of action, in so far as it is
understood by his own nature, is increased or diminished. Finally, by
perfection generally, I understand, as I have said, reality; that is to
say, the essence of any object in so far as it exists and acts in a
certain manner, no regard being paid to its duration. For no individual
thing can be said to be more perfect because for a longer time it has
persevered in existence; inasmuch as the duration of things cannot be
determined by their essence, the essence of things involving no fixed or
determined period of existence; any object, whether it be more or less
perfect, always being able to persevere in existence with the same force
as that with which it commenced existence. All things, therefore, are
equal in this respect.

_Definitions_

I.--By good, I understand that which we certainly know is useful to us.

II. By evil, on the contrary, I understand that which we certainly know
hinders us from possessing anything that is good.