The Greek philosopher Aristotle (384-322 B.C.) had defined humans as rational animals. The ability to observe, investigate, and understand causes and the capacity to act based on that understanding is essential to being human.
The Stanford Encyclopedia of Philosophy notes
But Grendel knows Unferth is "a thinking animal stripped naked of former illusions, stubbornly living on, ashamed and meaningless, because killing himself would be, like his life, unheroic." So Unferth is ancient Aristotle's human with a particularly modern problem. This thinking animal has figured out - with Grendel's help - that his thoughts are just thoughts and not grounded in any absolute reality that would make meaningful action possible.
Unferth has contracted a bad case of existential nihilism.
see also Philosophy of Aristotle