Whatever our souls are made of, his and mine are the same; and Linton’s is as different as a moonbeam from lightning, or frost from fire.
Catherine is trying to explain to Nelly a love that feels like recognition rather than romance. The line separates two bonds—one deep and essential, the other polished but thin—using bright, simple images. “Moonbeam” and “lightning,” “frost” and “fire” show contrasts you feel at once, which matches Catherine’s certainty. The device works because it translates an inner truth into clear pictures from nature, the world she knows best. The emotion is In Love, but it is also about identity—she feels Heathcliff is part of her selfhood. Many readers resonate with this idea: the person who feels like “home” isn’t always the safe or easy choice.