On April 26, 2018, Trump called in to Fox and Friends from the White House and spoke to the three hosts for around half an hour on live television. His 30-minute conversation was often a rambling, barely-coherent monologue. He jumped from topic to topic, covering Iran, Ronnie Jackson, Michael Cohen, Stormy Daniels, James Comey and Kanye West, often in just a few sentences.  He has done this on numerous occasions since entering the White House.
“You call places like Malaysia, Indonesia, and you say, you know, how many people do you have? And it’s pretty amazing how many people they have.” Donald Trump gave this rambling and strange answer as part of his response to a question on the corporate tax rate.  He has bragged that his "nuclear button" is larger than that of North Korea's Kim Jong-un.  There are very clear signs in his answers that he is unable to complete even simple thoughts and formulate rational responses to basic questions. This is precisely the same deterioration that was evident in Ronald Regan’s decline after he left the White House. He began using fewer and more simplistic adjectives over the course of his presidency. His sentences and thoughts were often left incomplete. He was later diagnosed with Alzheimer’s in 1994. Trump has not always been like this. Footage of him from a few years ago shows a man who was able to speak at length on a single subject. His stories had a clear structure and were highly coherent. His vocabulary was broader and he was more articulate, often using fewer superlatives and extreme adjectives.  There are also physical signs of neurological abnormalities. In one speech announcing the relocation of the American embassy to Jerusalem from Tel Aviv, his speech sounded slurred. In November, he also used his free hand to steady his other hand as he lifted a bottle of water to his lips, potentially suggesting that his motor functions may be impaired.
Counter 1 Everyone experiences deterioration in their motor and cognitive abilities after the age of 40. There is nothing abnormal about his behaviour. After 40, the brain shrinks by around 5% per decade. The first skills to be impacted are motor skills. However, the same brain deterioration also affects language, judgement and impulsivity. Trump’s behaviour has nothing to do with mental illness. He is simply suffering from the normal symptoms of ageing. Counter 2 You cannot come to an accurate diagnosis of Donald Trump's mental state from limited public appearances and tweets. It is impossible to reach a firm conclusion from an individual's public life alone. It is unfair to speculate about someone's mental state without conducting a psychiatric evaluation.
[P1] Signs of mental illness are clearly visible in his speech and behaviour. [P2] Therefore, Donald Trump has a mental illness.
[Rejecting P1] The signs described are not only characteristic of mental illness. They are also associated with ageing. [Rejecting P2] Even if they were, it is impossible to reach a conclusion of mental illness without a thorough evaluation.