A language is a dialect with an army and a navy. Sociologist and Yiddish scholar Max Weinreich recounted a conversation with an audience member of a lecture he gave in 1943. The audience member asked him about the difference between a language and a dialect. After Weinreich gave his explanation, the audience member responded, โA language is a dialect with an army and a navy.โ
This is published in the 1945 article Der YIVO un di problemen fun undzer tsayt (ืืขืจ ืืึดืืืึธ ืืื ืื ืคึผืจืึธืืืขืืขื ืคึฟืื ืืื ืืืขืจ ืฆืฒึทื โThe YIVO Faces the Post-War Worldโ; literally โThe YIVO and the problems of our timeโ).
ืึท ืฉืคึผืจืึทื ืืื ืึท ืืืึทืืขืงื ืืื ืึทื ืึทืจืืื ืืื ืคึฟืืึธื
a shprakh iz a dyalekt mit an armey un flot
The point of the retort is that the distinction between a language and a dialect is political rather than linguistic. Consider the linguistic and political differences between, for example, Spanish and Italian, compared to Moroccan Arabic and Iraqi Arabic, or Urdu and Hindi, or German and Yiddish.
Depending on the perspective, all languages on Earth are dialects of human language; on another perspective, British, American, Canadian Englishes could all be separate languages. A more realistic comparison would be that on an objective level, the linguistic features of romance languages such as French, Spanish, Italian, etc. are more similar than the linguistic features between different dialects of Arabic (Shami, Msri, Juba, Khaliji, etc). Though I canโt quite understand why that sense of pan-Arabic identity is supposedly strong enough to unite those dialects into one language; is not a pan-Romance identity just as strong? There must be more to the historical comparison between Arab and Romance language and identity development.