Here is a relevant excerpt: As you can see, it ends with "From this proposition it will follow, when arithmetical addition has been defined, that 1+1=2." The theorem above, ∗54 ⋅ 43 ∗ 54 ⋅ 43, is already a couple of hundred pages into the book (Wikipedia says 370 or so) I wrote a blog article a few years ago that discusses this in some ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.