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?

Solution 1:

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:

To emphasize participation by the reader and ensure that he or she is included.
To not sound egotistical. As in, to stress the mathematics and reduce the role of the author in particular.
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.

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