Why are proofs written in first person plural? Were they ever written differently? It's probably a silly question but it interests me when was the convention of writing proofs in first person plural introduced? Are here any historical examples of a different POV for proof writing?

From what I could gather from some google searches (1, 2, 3-pdf, and Krantz has a good book on writing mathematical prose) the most common reasons for using _we_ instead of _I_ are:

1. To emphasize participation by the reader and ensure that he or she is included.
2. To _not_ sound egotistical. As in, to stress the mathematics and reduce the role of the author in particular.
3. To keep the writing in active voice.



I couldn't find when a the shift in writing style occurred, but I do recall reading papers by Euler in which he uses _I_. Though, I cannot read Latin and these were translated.