The answer to this question can be found in Chapter Five, which is when the Grendel first meets the dragon in the pages of this excellent novel. According to the dragon, what distinguishes humans from other species is their complete inability to reason with logic and then their use of poetry and the Shaper to connect their badly concluded reasons together. Note his assessment of humans' logical faculties:
They only think they think. No total vision, total system, merely schemes with a vague family resemblance, no more identity than bridges and, say, spirderwebs.
Our complete inability to make decisions and rationally conclude anything is something that even we humans are apparently aware of, as at times we have the sneaking suspicion that all we live by is "nonsense," and this is where the Shaper provides humans with "an illusion of reality--puts together all their facts with a gluey whine of connnectedness." According to the dragon, therefore, we are stupid creatures who leap to impossible conclusions and are not able to logically reason anything.