The first type of progressivism has its philosophical underpinnings in 18th Century, Enlightement-era thought. It believes that politics is a battle of ideas. It further believes that through the use of reason and the exchange of ideas, human society will tend to improve itself through scientific and technological innovation. Hence, it believes in progress, and for this reason lays claim to the term “progressive”. Because of its belief and optimism in the faculties of human reason, I refer to this philosophy as rational progressivism.  – Nate Silver

This is as good as a place an any to start. This where I came from, and this is what I can no longer believe in.