To the moderators: I'm not sure this is the right place for this-or exactly where it should go. Please move to suit if needed.
Anyhow: in theory (so said William Safire) each cabinet ought to contain one future president. That hasn't really happened. Consider: the last cabinet officer to gain the Oval Office was Herbert Hoover; before him, William Howard Taft. Then one has to go back before the Civil War to find others.
Further: the presidential careers of those attaining the presidency after terms in the cabinet aren't all that stellar. Here they are, to the best of my knowledge:
Thomas Jefferson: lionized on Mt. Rushmore; a great small government theoretician; author of the Declaration of Independence and the Louisiana Purchase; policies toward Great Britain and freedom of the seas pushed the nation closer to war in 1812
James Madison: sort of prosecuted the War of 1812; again, another great theoretician but fairly light on accomplishments
James Monroe: two term president who lucked out by holding office during the Era of Good Feeling
John Quincy Adams: highly capable but seemingly always at odds with everyone
Martin van Buren: ward heeler written large
James Buchanan: ranks dead last among presidents, and deservedly so
William Howard Taft: about average among presidents; far better suited (by his own admission) to the judiciary
Herbert Hoover: great humanitarian and administrator; not so hot as executive (although in fairness a lot of what he started was co-opted and re-branded by Franklin Roosevelt)
So...does Safire's thesis hold up, or is it more pie in the sky?