Full Idea
The good is by no means relative to opinion, but is rooted in the unconscious and fatal nature of living beings, a nature which predetermines for them the difference between foods and poisons, happiness and misery.
Gist of Idea
The good is not relative, but is rooted in facts about human needs
Source
George Santayana (Platonism and the Spiritual Life [1927], p.3), quoted by John Gray - Seven Types of Atheism 6
Book Reference
Gray,John: 'Seven Types of Atheism' [Penguin 2019], p.130
A Reaction
That is, he concedes that the good is relative to human beings, but that the relevant facts about human beings are not relative. I think he has the correct picture. The key point is that the good is 'rooted' in something, and doesn't just float free.