Related to: How to Convince Me That 2 + 2 = 3
This started as a reply to this thread, but it would have been offtopic and I think the subject is important enough for a top-level post, as there's apparently still significant confusion about it.
How do we know that two and two make four? We have two possible sources of knowledge on the subject. Note that both happen to be entirely physical systems that run on the same merely ordinary entropy that makes car engines go.
First, evolution. Animals whose subitizing apparatus output 2+2=3 were selected out.
Second, personal observation; that is, operation of our sense organs. I can put 2 bananas on a... (read 208 more words →)
I've now added password reset capability to GreaterWrong.