The social state is at once so natural, so necessary, and so habitual to man, that… he never conceives himself otherwise than as a member of a body.

Mill is emphasizing that human beings are not naturally self-sufficient atoms but social creatures whose identity and thinking are formed within collective life. The “social state” is described as natural (rooted in human nature), necessary (required for survival and development), and habitual (so ingrained it becomes second nature). Because of this, people ordinarily imagine themselves as parts of a larger whole—“a member of a body”—rather than as isolated individuals. The remark underlines a tension central to Mill’s political thought: even when defending individual liberty, one must recognize that society powerfully shapes persons, and that social belonging is a default condition rather than an optional add-on.