The Ethics was a philosophical work by Benedict de Spinoza, written in Latin and published in the Netherlands in 1677.
Gary Mitchell considered this reading to be "longhair stuff" when James Kirk tried to interest him in it at Starfleet Academy. Infused with psionic energy at the galactic barrier in 2265, a mutated Mitchell viewed the work on a sickbay reader screen and found it "simple – childish almost."
Discussing Spinoza with Kirk, Mitchell had just been reading Part IV, the proof of Proposition XXXVII, which stated: "The good which every man, who follows after virtue, desires for himself he will also desire for other men, and so much the more, in proportion as he has a greater knowledge of God." (TOS: "Where No Man Has Gone Before")
The android Lal studied this book in 2366 as part of her instruction into the nature of humanity. (TNG: "The Offspring")
Appendices
Background
The 21st century translation of the Latin title drops the word "The" from the title, naming it simply "Ethics". The original Latin title is "Ethica".
For remastered TOS, certain pages from Ethics were considered to be added into the episode. VFX Line Producer Michael Okuda said of the literary work, "We tracked it down, we got the text and we really wanted to put it into the screen [....] We wanted to splash it on, frame by frame, to show Gary reading it, but... just couldn't make it happen." (Starfleet Access for "Where No Man Has Gone Before", TOS Season 1 Blu-ray special features)