I was reading a book and encountered a phrase in early modern English:
"The Gothes speake dowche, which I knowe by a dowcheman, my servunt, that was with me there : for they understode one an other well enough, as we understande a furlane {i.e., a man of Forli) or a florentine."
I have asked this question because grammatically and, most importantly, logically it is very coherent with the modern English, despite being orthographicly dissimilar. But the usage of the preposition "by" here is ambiguos to me. What is the meaning of "to know something by someone" and dowche? Is dowche the name of a language? I couldn't find its definition.