Wikipedia
John Feerick, the principal drafter of the 25th Amendment writes that Congress determined that "a rigid constitutional definition [of the terms unable and inability] was undesirable, since cases of inability could take various forms not neatly fitting into such a definition .
The debates surrounding the Twenty-fifth Amendment indicate that the terms unable and inability are intended to cover all cases in which some condition or circumstance prevents the President from discharging his powers and duties
A survey of scholarship on the amendment found no specific threshold – medical or otherwise – for the "inability" contemplated in Section 4.
The framers specifically rejected any definition of the term, prioritizing flexibility. Those implementing Section 4 should focus on whether – in an objective sense taking all of the circumstances into account – the President is "unable to discharge the powers and duties" of the office.
The amendment does not require that any particular type or amount of evidence be submitted to determine that the President is unable to perform his duties.
While the framers did imagine that medical evidence would be helpful to the determination of whether the President is unable, neither medical expertise nor diagnosis is required for a determination of inability.
To be sure, foremost in [the minds of the framers] was a physical or mental impairment. But the text of Section 4 sets forth a flexible standard intentionally designed to apply to a wide variety of unforeseen emergencies.
Here is the Presidential Oath of Office.
"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
I think the particular circumstance of President Trump's incitement of insurrection is a naked manifestation of gross unfitness to remain in office that is tantamount to a demonstration of his inability to "preserve, protect and defend the Constitution of the United States," more than sufficient grounds for his removal under the 25th Amendment.