I find it really amazing that this is so complicated! It is my opinion that porter is THE most written about and researched "style" ever! It seems almost ever brewing book I have read has something about porter in it. Off the top of my head I think all of the following Classic beer style series books have at least some mention of the influence of porter. Barley wine, Mild ale, Pale ale and Scotch ale and of course Porter. The last one is the only one I have not actually read! Some of them have rather extensive passages about the popularity and rise and fall of said "style". There loads of historical info on the style in Radical Brewing as well. How can something so well documented be so contentious?
Could be because it came about when "styles" were really beginning to be documented? Or rather when breweries/brewers were really beginning to market there beer and they threw names around willy nilly?
I agree with the folks who mentioned the use of black malt (patent) as a distinuisher of at least modern day porter vs. a stout which uses roast barley.
And Guinness Stout is a good example of how things evolve and mutate...evidently roasted barley wasn't even part of the grist until well into the 20th century.
Don't forget that Guiness was originally known for their porter
which they discontinued sometime in the early 20th century iirc. What I recall is that they started brewing a "Stout porter" which was a variation on their porter. Stronger? I dunno. Blacker? I dunno. Stouter? uh, sure... Whatever that means. Marketing term to mean more special therefore cammanding a premium price???
If you look at early 20th century beer ads and marketing they used not only the terms stout and porter, but also beer and ale interchangeably. No wonder it gets so confused.
Perhaps I missed it but I do not seem to recall anyone in this thread or the article linked mentioning the much documented theory that porter itself was not actually something brewed as is but rather a blend of beers. Also a likely early marketing ploy. A way to use up beer that had gone by with beer that was not quite ready to drink.
If you accept that then really no one is making "authentic" porter anymore. It would not make much sense on a commercial level.
It is also my understanding that when the beer that came to be known as porter which was now being brewed "whole" rather than blended used a variety of substances to darken it; including some rather dubious ones, until a reliable method for making roast malt (black "patent") was developed.