This quote appears when Gulliver meets the inhabitants of Laputa, a floating island where "great" minds gather to think, study, and experiment. The qualifying word is in quotations because Gulliver soon discovers that although the Laputians consider themselves extremely intelligent, they are almost incapable of mundane thoughts and tasks, and need servants to help them through every non-intellectual task.
Their heads were all reclined, either to the right, or the left; one of their eyes turned inward, and the other directly up to the zenith.
It seems the minds of these people are so taken up with intense speculations, that they neither can speak, nor attend to the discourses of others, without being roused by some external action upon the organs of speech and hearing...
(Swift, Gulliver's Travels, eNotes eText)
Their heads are inclined and their eyes pointing in different directions to symbolize their philosophical thoughts; the eye pointing "inward" refers to deep personal contemplation, while the eye pointing "to the zenith" refers to consideration of the larger world. Both are parodies of real philosophical thinking; the Laputians can barely function in the real world, making personal contemplation pointless, while they only consider the larger world in abstract rather than pragmatic terms. The Laputians serve as satirical versions of the pseudo-intellectuals that Swift despised.