So Eliot was sensitive to certain manifestations of the uncanny, and to terrors that might well cause shuddering. We have now to ask a more difficult question: why did those lines of In Memoriam affect Eliot so exceptionally, move him to use ‘shudder’ as a laudatory critical term? Of course we may say that Victor Hugo had already done this when he told Baudelaire that in writing Les Fleurs du mal he was creating ‘un frisson nouveau’. And many of us remember the days when ‘the metaphysical shudder’ was a stock term in discussions of Donne and his contemporaries. But in the In Memoriam passage the shudder is not a metaphysical shudder. In what is as far as I know his only essay on Tennyson, Eliot certainly intends praise. His first sign of enthusiasm is for a line or a fragment of a line from ‘Mariana’, which, he thinks, offers something ‘wholly new’: ‘The blue fly sung in the pane.’ So far the youthful Tennyson has excelled as a metrist and indeed is credited with most of the attributes of greatness without quite deserving to be called ‘great’. Yet, Eliot says, this line or fragment ‘is enough to tell us that something important has happened’. Now the line has many clear merits, it conveys a recognisably melancholy, apprehensive mood of waiting; and it may conceivably be true to say, as Eliot does, that its effect depends finally on reading ‘sung’ for ‘sang’. The line belongs to a good poem, rightly praised as a whole; but there was still something distinctive about it: it got itself chosen. The only explanation offered is that ‘something important has happened.’
more from Frank Kermode at the LRB here.