George H. Smith
George H. Smith was formerly Senior Research Fellow for the Institute for Humane Studies, a lecturer on American History for Cato Summer Seminars, and Executive Editor of Knowledge Products. Smith's fourth and most recent book, The System of Liberty, was published by Cambridge University Press in 2013.
An Introduction to Libertarianism.org.
You can think of negative liberty as being about the absence of external limits, and positive liberty as the absence of internal limits.