@Blaergh If you would like to avoid the nowadays unusual notation of Principia Mathematica, you could give #Metamath a try. For instance, 2p2e4, link is below, is the proof for 2+2=4. It allows you to track thos down to the axioms of set theory. As there are so many people trying to convince me that 2+2=5, this is an invaluable resource, IMHO.
@Blaergh If you would like to avoid the nowadays unusual notation of Principia Mathematica, you could give #Metamath a try. For instance, 2p2e4, link is below, is the proof for 2+2=4. It allows you to track thos down to the axioms of set theory. As there are so many people trying to convince me that 2+2=5, this is an invaluable resource, IMHO.
https://us.metamath.org/mpeuni/2p2e4.html