 Berlin’s best-known contribution to political theory is his essay on the distinction between positive and negative liberty. This distinction is explained, and the vast literature on it summarised, elsewhere in this encyclopedia; the following therefore focuses only on Berlin’s original argument, which has often been misunderstood, in part because of his own ambiguities. It should be stressed that the essay in question is principally concerned with political liberty, not with what, late in life, he dubbed ‘basic liberty’, which is freedom of choice (or free will), without which any other kinds of liberty would be impossible: indeed, ‘which men cannot be without and remain men’ (A 518; cf. UD 218, CTH2 309, and 2.4 above).