There is nothing good or bad by nature, for if there is anything good or bad by nature, it must be good or bad for all persons alike, just as snow is cold to all. But there is no good or bad which is such to all persons in common; therefore there is no such thing as good or bad by nature. For either all that is thought good by anyone whatever must be called good, or not all. Certainly all cannot be so called; since one and the same thing is thought good by one person and bad by another; for instance, Epicurus thought pleasure good and Antisthenes thought it bad; thus on our supposition it will follow that the same thing is both good and bad. But if we say that not all that anyone thinks good is good, we shall have to judge the different opinions; and this is impossible because of the equal validity of opposing arguments. Therefore the good by nature is unknowable.
Source: Lives of the Eminent Philosophers (1925) by Diogenes Laërtius, translated by Robert Drew Hicks. A Loeb Classical Library edition; volume 1 published 1925; volume 2 published 1925. WikiSource.