The following text is adapted from Oscar Wilde's 1897 nonfiction work De Profundis.

People whose desire is solely for self-realisation never know where they are going. They can't know. In one sense of the word it is of course necessary to know oneself: that is the first achievement of knowledge. But to recognise that the soul of a man is unknowable, is the ultimate achievement of wisdom. The final mystery is oneself. When one has weighed the sun in the balance, and measured the steps of the moon, and mapped out the seven heavens star by star, there still remains oneself. Who can calculate the orbit of his own soul?

Which choice best describes the function of the underlined question in the text as a whole?