The danger, when not seen, has the imperfect vagueness of human thought. The fear grows shadowy; and Imagination, the enemy of men, the father of all terrors, unstimulated, sinks to rest in the dulness of exhausted emotion.
Imagination is called the "enemy of men" because it can torment us with frightening possibilities. Human thought is imperfect and vague when confronted with unseen dangers. The constant strain of fear and anxiety can drain our emotional resources. The psychology that Conrad writes about is profound.
