"one of their Eyes turned inward, and the other directly up to the Zenith"
At that time, western Europe was enjoying the Age of Reason. Science was in the ascendant, and Isaac Newton (1642 - 1727) was the hero of the age. Swift, whose intention was to expose all the follies of mankind, did not hesitate to take on Reason. He did not chastise Reason so much as undue faith in it; he deplored making a religion of Reason to the detriment of religious faith, morality and ethics.
With one eye turned inward and the other skyward, Swift points to the two great scientific instruments of the age, the microscope and the telescope.