The opening stage direction in the play Remember Caesar by Josephine Tey (writing as "Gordon Daviot") includes the paragraphs:
LORD WESTON is seated by the fireplace, a table of books and papers beside him. He is engaged in filling his pipe. And talking.
Down right, where the light from the side window falls across his small writing-table, is seated MR. ROGER CHETWYND, a thin, earnest, absent-minded, and conscientious youth. So conscientious is he that his mind, even when absent, is absent on his employer's [i.e., Lord Weston's] business. He has begun by listening to his master's lecture, but the lure of his work has been gradually too much for him, and he is now blissfully copying from one paper on to another while the measured words flow over him, his lips forming the phrases while he writes.
What is the meaning of the sentence in bold?