Mr. Adler's latest book is We Hold These Truths, an examination of the American Constitution, understood as comprising the Declaration of Independence, the Constitution itself, and - the Gettysburg Address. The discussion, as always with this favorite Firing Line guest, is rich but never abstract; and if Mr. Buckley and many of his fans will disagree with Mr. Adler at times - well, even Homer nods. MA: "In every other constitutional system I know ... there's a separation between the head of state and the chief of government - which we don't have. We combine head of state and chief of government in the same man and therefore give, symbolically, much more gravity to the Presidency than it should have.... I've often said that we ought to have a front-parlor President who speaks very well, the way Reagan does, and a back-parlor President who thinks better than Reagan does."