tablets of his mind
Trollope is referring here to Mr. Gresham’s views on the subject of whether or not Frank needs to marry a person who is wealthy. The squire himself likes Mary Thorne, with whom Frank is in love, but the De Courcy relatives, along with Lady Arabella, feel that Frank needs to marry money in order to save the Greshamsbury estate. Trollope says that the De Courcy family has not engraved this idea on the tablets of Mr. Gresham’s mind–in other words, he does not share their beliefs on this subject. We find this turn of phrase in Aeschylus’ Prometheus Bound. Before Prometheus prophesizes to Io about her future, he tells her: “write it in the tablets of your mind.” [MD & RR 2005]
Sources: Aeschylus, Prometheus Bound 788-789.