"Who knows?"

On the penultimate page of David Lodge's novel Paradise News, Bernard reads a quotation from Miguel de Unamuno's The Tragic Sense of Life, sent to him from Hawaii by the woman whose car knocked his father down:

In the most secret recess of the spirit of the man who believes that death will put an end to his personal consciousness and even to his memory forever, in that inner recess, even without his knowing it perhaps, a shadow hovers, a vague shadow lurks, a shadow of a shadow of uncertainty, and while he tells himself: "There is nothing for it but to live this passing life, for there is no other!" at the same time he hears, in his most secret recess, his own doubt murmur: "Who knows?..." He is not sure he hears aright, but he hears.
      Likewise, in some recess of the soul of the true believer who has faith in the future life, a muffled voice, the voice of uncertainty, murmurs in his spirit's ear: "Who knows?..." Perhaps these voices are no louder than the buzzing of mosquitoes when the wind roars through the trees in the woods; we scarcely make out the humming, and yet, mingled with the roar of the storm, it can be heard. How, without this uncertainty, could we ever live? [p. 293 (of Paradise News)]