During much of its history, and especially during the Francoist dictatorship (1939–1975), the Catalan language has often been degraded as a mere dialect of Spanish. This view, based on political and ideological considerations, has no linguistic validity. Spanish and Catalan have important differences in their sound systems, lexicon, and grammatical features, placing the language in a number of respects closer to Occitan (and French). 

What has Catalan often been considered?