The use of the word "oneness" in this question is particuarly interesting, as this is a word that Arnold himself uses to describe the figure of the scholar-gypsy, who is a figure that, in his mind, has entered immortality precisely because of his ability to simply focus on one thing in life rather than become distracted and worn down by focusing on many different areas, which, Arnold feels, is the mistake made by man in his time. Note how Arnold praises the scholar-gypsy for his single-minded focus in the following quote:
Thou hadst one aim, one business, one desire;
Else were thou long since numbered with the dead!
Else hadst thou spent, like other men, thy fire!
Arnold's use of the italics in emphasising the repeated word "one" clearly indicates the way in which, to his mind, the scholar-gypsy possessed the oneness in life that is focused on in this question and which has been lost by humans. Note how the quote develops the contrast between the oneness of the scholar-gypsy and the disorientated focus of other humans, who have "spent" their fire and are "numbered with the dead." Arnold suggests that it is the oneness of the scholar-gypsy that has allowed him to survive, whereas giving in to the "strange disease of modern life / With its sick hurry, its divided aims" is the reason why humans die young, exhausted and frustrated.