Sex Ratio Theory, Ancient and Modern - An 18th Century Debate about Intelligent Design and the Development of Models in Evolutionary Biology [pdf file]. The design argument for the existence of God took a probabilistic turn in the 17th and 18th centuries. Earlier versions, such as Thomas Aquinas’ 5th way, usually embraced the premise that goal-directed systems (things that “act for an end” or have a function) must have been created by an intelligent designer. This idea – which we might express by the slogan “no design without a designer” – survived into the 17th and 18th centuries, and it is with us still in the writings of many creationists. The new version of the argument, inspired by the emerging mathematical theory of probability, removed the premise of necessity. It begins with the thought that goal-directed systems might have arisen by intelligent design or by chance; the problem is to discern which hypothesis is more plausible. From Professor Elliott Sober.