Just to add some more fun quotes:

In dividing official from unofficial conduct, courts may not inquire into the President’s motives. Such a “highly intrusive” inquiry would risk exposing even the most obvious instances of official conduct to judicial examination on the mere allegation of improper purpose. Fitzgerald, 457 U. S., at 756. Nor may courts deem an action unofficial merely because it allegedly violates a generally applicable law.

This case poses a question of lasting significance: When may a former President be prosecuted for official acts taken during his Presidency? In answering that question, unlike the political branches and the public at large, the Court cannot afford to fixate exclusively, or even primarily, on present exigencies. Enduring separation of powers principles guide our decision in this case. The President enjoys no immunity for his unofficial acts, and not everything the President does is official. The President is not above the law. But under our system of separated powers, the President may not be prosecuted for exercising his core constitutional powers, and he is entitled to at least presumptive immunity from prosecution for his official acts. That immunity applies equally to all occupants of the Oval Office. P