27 psychologists, psychiatrists and mental health experts shared their diagnoses in a book entitled 'The Dangerous Case of Donald Trump'. They did so out of a sense of civic duty to warn the American public of the imminent danger their unstable president poses.
Mental health experts have suggested the president may be suffering from: Malignant Narcissism  Symptoms include: A lack of self-awareness, entitlement, lack of authenticity, a need for control, a lack of concern over the consequences of their behaviour, and a desperate need for the approval and attention of others. Antisocial personality disorder Symptoms include: Extreme arrogance, persistent lying, often breaking the law, a disregard over the safety or interests of others, a lack of guilty feelings, and frequent outbursts of anger. Alzheimers or another cognitive disease   Symptoms include: The inability to finish thoughts or sentences, the inability to maintain a train of thought, abandoning the use of complex vocabulary, and memory problems. Donald Trump has repeatedly exhibited symptoms of each of these disorders and is doing so at a more frequent rate. He is lying more than he did at the start of his presidency. His rallies have become more fervent. His speeches are more disjointed and incoherent. He has also become more impulsive, often making decisions against the guidance of those around him (his latest drive to have Obamacare thrown out was opposed by several cabinet members).
There are also many mental health professionals that do not believe Trump is displaying signs of mental illness. Dr. Robert Pyles, a Boston-based psychiatrist has publicly insisted that he has seen no "evidence of neurological or cognitive dysfunction." Many of the "symptoms" psychologists allude to are part of Donald Trump's strategy. He uses simpler words and vocabulary to appeal to his base, who are usually non-college educated voters and appreciate straight-talking, simple messaging.
[P1] Mental health professionals believe Trump is mentally ill. [P2] Therefore, it is reasonable to conclude he is.
Rejecting the premises
[Rejecting P1] Plenty of mental health professionals argue these conclusions.