This is Balfour's Foundations of Belief

This contains an argument very similar to Lewis's AFR.

Balfour's Theism and Humanism is online

A book Lewis said was "too little read." My dissertation adviser Hugh Chandler first notice the close relationship between Lewis's AFR and Balfour's by reading a Moore essay criticizing Balfour.

Arthur Balfour's Dangerous Idea

Balfour is one of the early forefathers of the argument from reason, and we know that Lewis read and recommended Balfour. It was my dissertation advisor, Hugh Chandler, who discovered the connection between Balfour and the AFR, and later game me a copy of The Foundations of Belief he found in England. This post, by Jim Slagle, who wrote his master's thesis on the AFR, links to an online edition of Balfour's first philosophical book, A Defense of Philosophic Doubt, published in 1879, and my be the first post-Darwin version of the AFR to come out.

Arthur Balfour online

Arthur Balfour, the former British Prime Minister and one of the founding fathers of the AFR, has a couple of books that can be read online. Actually The Foundations of Belief can be read online as well, as I linked to it on DI2.