The speaker feels stuck between thinking and living. The "sages in my head" are probably the voices of all the wise people he has read, and instead of comforting him they trouble him. His books, which should be a source of warmth or answers, have turned to ice: cold, hard, and no use to touch. The title, "Heretic," suggests he may be breaking with this inherited wisdom, or at least doubting it.
The second stanza shifts from thought to feeling. Evening arrives and brings "an absence," a sense that something is missing. The comparison is striking: the absence is "as old as a horse in the rain." We can picture an aging animal standing patient and soaked, not seeking shelter. That image carries weariness, loneliness, and quiet endurance.
The mood is bleak but calm. Nothing dramatic happens. The poem simply sits with a lingering emptiness, one that learning alone can't fill.