How or where can I learn more about the US constitution, its practice and the philosophy behind it?
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The US Constitution is literally ancestor worship anon, the same as Japanese Shinto.
In order to practice proper representative democracy you need a concept such as Parliamentary Sovereignty where dead men can't bind their successors and take away the freedom of the living.
He's just buying in to the whole "monarchy is cool because it allows the focus of nationality to exist outside politics" meme. If he weren't he probably would have brought up the Magna Carta instead of the Glorious Revolution.
Your so-called prime ministers are beholden first to their colleagues in parliament, as Jeremy Corbyn is finding out as opposition leader. No amount of public support will save him. In a system where the executive is also required to be the strongman of parliament, no true tribune of the people can emerge. This is why the American system of selecting a sovereign is superior.
The PM exercises executive power - all the monarch's power has been sucked away. In the US the president, in practice, fills the role of a sovereign. He directs the machinery of the state in accordance with the law.
Pretty much this. It was hastily put together, and relied almost entirely on last-minute compromise in an effort to keep the country from imploding. None of the founding fathers thought it would last very long, and the only reason it has is because of hero worship and the weird notion that the founding fathers were infallible and created something perfect.