What is the origin of the doctrine of perspicuity?

Fundamentally, Catholics and Protestants read Scripture differently. Catholics believe that any private interpretation of Scripture is subject to the teachings of the church, whereas Protestants, subscribing to the priesthood of all believers, afford more latitude for private interpretation. Within that "private interpretation," however, is the doctrine of [perspicuity](http://www.theopedia.com/Clarity_of_Scripture) – a term that ironically means that Scripture is always clear. (Ask any congregation to define it, and I'll bet 75% can't!) The question, however, is how this doctrine developed. Is it Scriptural, for example, to say that the Bible proclaims itself as having obvious application? (The Gospel is veiled to those who are perishing) Or is it more of a historical reaction? (The answer to Pope is not to install a new Pope) In other words, where does this doctrine come from?