“In the four principal Parts there ought to be four Performers of almost equal Mastery […]. But this Rule is generally neglected by placing one of the worst Hands on the Tenor; which, though a Part of little Execution, yet requires so much Meaning and Expression, that the Performer should not only give a fine Tone, (the peculiar Quality of that Instrument) but by swelling
and singing of the Notes, and entering into the Spirit of the Composer, know, without destroying the Air, where to fill the Harmony; and, by boldly pointing the Subject, keep it up with the greatest Energy”
Charles Avison, An Essay on Musical Expression, London, 1753, pp. 132/133