1/2 A mystery for those interested in the history of history teaching, and something I'd love some help in! Where was the concept of "similarity and difference" originally defined/proposed as a distinct concept? Can't find a root source anywhere!

2/2 A myriad of different pieces of literature point to the 1970s SCHP, but can find little literature on the new concepts they defined, and where there is literature, no reference to simmilarity/difference - strange. Welcome suggestions.