Many types of `other-regarding' acts and beliefs cannotbe accounted for satisfactorilyas instances of sophisticated selfishness, altruism,team-reasoning, Kantian duty, kinselection etc. This paper argues in favour ofre-inventing the notion of solidarity as ananalytical category capable of shedding importantnew light on hitherto under-explainedaspects of human motivation. Unlike altruism andnatural sympathy (which turn theinterests of specific others into one's own), orteam-reasoning (which applies exclusivelyto members of some team), or Kantian duty (whichdemands universalisable principlesof action), the essence of solidarity lies in thehypothesis that people are capable ofresponding sympathetically to (or empathising with)a condition afflicting ‘others’,irrespectively of who those others are or whetherone cares for them personally. Andwhen that condition is a social artefact, we argue,solidarity turns radical.