from Justice Souter's opinion in the Kentucky Ten Commandments case (paragraph break added):

The Counties' second proffered limitation can be dispatched quickly. They argue that purpose in a case like this one should be inferred, if at all, only from the latest news about the last in a series of governmental actions, however close they may all be in time and subject.

But the world is not made brand new every morning, and the Counties are simply asking us to ignore perfectly probative evidence; they want an absentminded objective observer, not one presumed to be familiar with the history of the government's actions and competent to learn what history has to show. The Counties' position just bucks common sense: reasonable observers have reasonable memories, and our precedents sensibly forbid an observer "to turn a blind eye to the context in which [the] policy arose."