Ah the wondeful De Sade,,, I have no doubt this great libertine writer indeed played both sides of the fence. I wonder though just when in history the term "switch" first developed. So I looked it up.
My boolean searches leave much to be desired on the topic. Most internet recources revealled nothing about when the term came to describe a crossection of the bdsm subculture. The earliest references I was able to find were about the old gaurd leatherman comunity that became prevelent after ww2, where switches were described as "switch hitters" going from being the penetrator to the penatratee, or as later described in the early sixties as going from "top" to "bottom". This seems to be the first real refferences to switches as a general catagory at least that i was able to dig up.
Switching thechnically happens all the time in everyones daily lives depending on who and what you are doing or dealing with. I am certianly more dominant when i am teaching a class in school, and more submissive when getting a speeding ticket etc, lol.