 The problem with this was that it wasn’t clear what could replace the best systems theory. Lewis floated two approaches in the postscripts to the reprinting of (1980a), one based on primitive chances, and the other based on history-to-chance conditionals being necessary. But neither seemed metaphysically plausible, and although each was consistent with the Principal Principle, they made it either mysterious (in the first case) or implausible (in the second). A better response, as set out in “Humean Supervenience Debugged”, was to qualify the Principal Principle. Lewis said that what was really true was the “New Principle”. His proposal was based on ideas developed by Ned Hall (1994) and Michael Thau (1994).