Article I, Section 3, Clause 6:"The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present."Got that?
When the PRESIDENT OF THE UNITED STATES is tried... Roberts is simply saying they are not trying the president, but a FORMER president and now private citizen, and therefore he has no constitutional duty to preside over that trial.
A view which is defensible, and which ought to be shared by everyone involved in attempting this trial. Trump is not president, and therefore this trial is unconstitutional.
