Pg. 826 of David Markson’s copy of The Complete Greek Drama: Volume One by Various (Ed. Whitney J. Oates & Eugene O’Neill, Jr.):

     On which Markson underlined part of a speech by the eponymous character in Euripides’ Hecuba:
     “I may be a slave and weak as well, but the gods are strong, and custom too which prevails o’er them, for by custom it is that we believe in them and set up bounds of right and wrong for our lives. Now if this principle, when referred to thee, is to be set at naught, and they are to escape punishment who murder guests or dare to plunder the temples of gods, then is all fairness in things human at an end.”

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     “The first writer known to condemn slavery is Euripides.”
     Markson notes on pg. 175 of his novel Reader’s Block.