His Storia critica di Spagne e della cultura spagnuola in ogni genere (2 vols., 1781–1784) was finally expanded into the Historia crítica de España y de la cultura española (1783–1805), which, though it consists of twenty volumes, was left unfinished; had it been continued on the same scale, the work would have consisted of fifty volumes. Masdeu wrote in a critical spirit and with a regard for accuracy rare in his time; but he is more concerned with small details than with the philosophy of history. Still, his narrative is lucid, and later researches have not yet rendered his work obsolete.
