Any source. Surveying all the evidence is a fundamental aspect of Marxist analysis.

It doesn’t have to be in English, either. It’s the language of the forum but there are people here from all over, who may be able to explain sources in other languages for those who can’t read it (and there’s always DeepL). That said, in this case, English is probably your best bet for a wider response and is probably the language of the source in any event.