"[M]y reference to an "invisible" Constitution should not be confused with the way some... are prepared to disregard all or part of the Constitution in times of crisis, real or manufactured. My interest... is in what... the Constitution's words cannot hope to reveal. I'll leave for another day the views of some that—because the Constitution is not... a "suicide pact"—we should sometimes act as though parts of what it says have conveniently become invisible."