Is that because all the voices are equalin nature, since it was written for the harpsichord (and virginal and whatever)?And in a fugue the main theme just happens to pop up every now then in the different registers - it would be polyphonic even if it didn't replicate the theme over and over?Anyway, as long as it isn't a chorale etc. I can be quite sure it's polyphonic?
This may be a question of distinguishing between texture and compositional technique.Polyphony is a musical texture.https://en.wikipedia.org/wiki/Texture_%28music%29Fugue is a specific type of compositional technique that is characterized by its contrapuntal or polyphonic nature.For non-fugue works try looking at: French Suites, English Suites, Partitas, or Toccatas. Note there may be non-labeled fugues within the individual movements.
You don't have to be such an arsehole about it.
I beg your pardon? Your initial remarks were not clear, and I was attempting to be helpful.
Just to be annoyingly pedantic (pedantry on Piano Street !?! Never!!! ): 'Sinfonia' is not usually used as a name for the 2-part Inventions, only the 3-part: the 2-part Inventions are just called Inventions.
Given your evident confusion regarding what constitutes polyphony in music, I am assuming you have no experience playing any of it (or at least playing it with an appreciation of what you are supposed to be looking for or doing). That being the case, I seriously suggest you look at some of the Sinfonias first, otherwise called the two part inventions. Having only two voices, and being designed as teaching aids in polyphony, they make an excellent starting point, and should be technically reasonably straightforward, allowing you to concentrate on the polyphony rather than the mechanics.