The dreamer can know no truth, not even about his dream, except by awaking out of it.

Santayana’s line uses the dream as a model for any immersive state—illusion, ideology, passion, or unexamined habit—in which one is absorbed and therefore unable to judge it from within. “Truth” here is not merely factual accuracy but reflective knowledge: to know what one is experiencing, and what it means, requires a standpoint outside the experience. The paradox is that the dreamer cannot verify even the contents of the dream while dreaming; only waking provides the critical distance needed to recognize it as a dream and to assess it. The remark thus underscores Santayana’s skepticism about self-certainty and his emphasis on detachment, perspective, and awakening into critical consciousness as conditions for genuine knowledge.