Upon first emerging from the cave Rabbe Shimon bar Yochai (Rashbe) and his son could not tolerate people who were plowing and planting (i.e. working to earn a livelihood). They could not make peace with the fact that these people were, "manichim chayay olam v'oskim b'chayay sha'ah - neglecting eternity and preoccupying themselves with transience." A heavenly voice instructed them to return to their cave. Upon reemerging a year later they encountered a man who was running with two bundles of hadasim (myrtle branches) right before Shabbos. Upon questioning the man explained that the two bundles corresponded to zachor and shamor. Hearing this Rashbe and his son were reassured and became at peace with the world.
How did this encounter with the old man reassure them? Rav Chayim of Volozhin in his Nefesh Hachaim (I:8) indicates that Rashbe agrees with Rabbe Yishmoel (RY) that the overwhelming majority of people should combine earning a livelihood with the study of Torah. Clearly neither Rashbe nor RY are conceding that it is a necessary evil to divert time and energy away from avodas Hashem to earn a livelihood. Rather they teach that it is Hakadosh Baruch Hu's will that people adequately address their mundane needs. Having been addressed the mundane fades into the background and people can devote themselves to the spiritual (see Rashi to Berachos 35b that one who neglects earning a livelihood and has to seek help will end up neglecting his Talmud Torah.)
One who is involved with the physical and mundane because these concerns define his life is guilty of being, "manni'ach chayay olam v'oseik b'chayay sha'ah - neglecting eternity." But one whose involvement is measured, framed, and contextualized by his ultimate overarching spiritual goal is involved with eternity at all times.
Shabbos is me'ein olam habboh, a miniature of the spiritual, eternal world to come. The old man running lekavod Shabbos - no simple task at his advanced age - reassured Rashbe and his son that their fellow Jews' real and ultimate preoccupation was the Eternal and thereby eternity.