Why is Decoud unequal to the challenge of solitude? There is an allegorical aspect
to this, in that the concept of skepticism can really only exist through comparison
with its opposite. It is wholly a negating concept, which must have something
to negate. Psychologically and allegorically, Decoud was defined in terms of opposition
to Antonia (idealism); see our first view of him through Antonia's eyes.
As long as Decoud was surrounded by others whose ideals he can sneer at, his skepticism
had fuel, but when left alone it turns to negating itself. This is the "grapple
with himself" that he is not fit to withstand.