Love is the condition in which the happiness of another person is essential to your own.

This line is widely attributed to Robert A. Heinlein and is commonly cited from his 1961 science‑fiction novel *Stranger in a Strange Land*. In the book, Heinlein uses aphoristic definitions and quasi-philosophical dialogue to explore human relationships, sexuality, religion, and the ethics of community through the perspective of Valentine Michael Smith, a human raised by Martians. The quote circulates as one of the novel’s compact “definitions” of love, reflecting the era’s mid‑century interest in psychology and selfhood as well as Heinlein’s recurring theme that mature affection involves responsibility and mutual regard rather than mere sentiment.

Heinlein’s formulation treats love less as a feeling and more as a relational condition: your well‑being becomes interdependent with another’s. The emphasis is not on possession or desire but on the integration of another person’s flourishing into your own sense of happiness. Read charitably, it describes empathy and commitment—an orientation in which another’s joy and safety matter intrinsically, not instrumentally. It also implies vulnerability: if your happiness depends on another’s, love exposes you to loss and requires care. The definition has endured because it is simple, ethically charged, and applicable across romantic, familial, and communal bonds.

1) “Love is that condition in which the happiness of another person is essential to your own.”
2) “Love is the condition where the happiness of another is essential to your own.”
3) “Love is the condition in which someone else’s happiness is essential to your own.”