The point of the
proposal language is not the Latin but the Oxford context. As Barbara
Reynolds writes in her 1993 biography, Dorothy L. Sayers: Her Life
and Soul, Dorothy wrote her friend Catherine Godfrey in 1913 about
getting tickets for the degree-granting ceremony at Oxford. She noted
that the format called for the Vice-Chancellor to address all the assembled
Oxonian doctors "in a sing-song little speech, beginning something about
'Does it please you doctors of the University that so-and-so should
be admitted to such-and-such a degree -- placet-ne?' and then
he took off his cap; then said 'placet' without leaving time
for anyone to make an objection if he wanted to, and put it on again."
And of course, Sayers
herself was one of the first group of women to be given the title of
Oxford magistra -- Master (of Arts) -- in the first degree-granting
ceremony for women in 1920. Harriet Vane as well would have been a domina
(Bachelor of Arts) and then after the passage of 5 years from her exams,
a magistra.
Put all this Oxford
background together and those three words in Latin of Peter's proposal
and Harriet's acceptance summarize the whole point of the book -- only
once he addresses her as his equal, as an Oxford magistra, in a totally
Oxonian idiom that belonged to them both as Oxford graduates, does she
feel comfortable enough as his equal to accept the proposal. "Placetne
magistra?" "Placet" sums up the whole emotional content of the book
in 3 words.