All foreign leaders who meet the US President these days have to ask themselves just one, deeply worrying question: does this man have a clue what he's talking about?
Or, to put it another way: Does he have the mental capacity to do the job he was elected to do?
I have witnessed a lot of press conferences during my 45-plus years as a reporter, but never, ever, have I witnessed anything to compare with President Trump's performance yesterday.
The Wall Street Journal, owned by Rupert Murdoch, summed it up perfectly: 'President Donald Trump defended as highly effective his tenure so far in the White House, which has been marked by legal fights, West Wing power struggles, confrontations with US allies, the withdrawal of one of his cabinet nominees and the firing of his national security adviser after he misled administration officials about his contacts with Russia.'
(Within hours, the man he had picked as his new national security adviser was reported to have turned the job down, apparently having described the prospect of joining an administration that Mr Trump insists is a finely-tuned machine as a 'shit sandwich'.)
This is the man who was reported last summer to have asked a foreign policy expert not once but three times: 'If we have them [nuclear weapons], why can't we use them?'
The man who was reported last week to have phoned his now ex-national security adviser at three o'clock in the morning to ask if a strong dollar or a weak dollar was better for the US economy.
The man who was reported this week -- again by the Wall Street Journal -- to be regarded with such deep suspicion by his own intelligence services that they have decided not to pass on everything they know because they don't trust him.
And the man who, with the Israeli prime minister Benjamin Netanyahu standing next to him, answered a reporter's question about the rise in anti-Semitic attacks since his inauguration with the words: 'Well, I just want to say that we are, you know, very honoured by the victory that we had -- 316 electoral college votes ... As far as people, Jewish people, so many friends; a daughter who happens to be here right now; a son-in-law, and three beautiful grandchildren. I think that you're going to see a lot different United States of America over the next three, four or eight years. I think a lot of good things are happening. And you're going to see a lot of love. You're going to see a lot of love. OK? Thank you.'
A day later, he was asked the same question again, and after calling the question unfair and insulting, these were his exact words: 'Here's the story, folks. Number one, I am the least anti-Semitic person that you've ever seen in your entire life. Number two, racism. The least racist person ... I hate the charge. I find it repulsive. I hate even the question.'
There is nothing new about suggesting that Donald Trump might not have the ideal temperament to be the head of state of the most powerful nation on the planet. What is new is the growing sense that he might not have the mental capacity. In the latest edition of the New York Review of Books, the highly respected commentator Elizabeth Drew writes: 'Trump's possible mental deficiencies are ... a troubling question: serious medical professionals suspect he has narcissistic personality disorder, and also oncoming dementia, judging from his limited vocabulary. (If one compares his earlier appearances on YouTube, for example a 1988 interview with Larry King, it appears that Trump used to speak more fluently and coherently than he does now, especially in some of his recent rambling presentations.)'
The fact that Mr Trump is determined to wage war on the media -- except those that are uncritical of him -- is not the most serious of his many shortcomings. What must, surely, be far more worrying to every sentient being in Washington and around the world is that he appears to have only the most tenuous grip on reality.
He insists that he won the biggest election victory since Ronald Reagan, and when he is told to his face that he didn't, he sulks like a schoolchild: 'Well, I was given that information.'
I was in good company as my jaw hit the floor as I watched him in full flow at his press conference, reduced at one point to insisting (despite all the evidence to the contrary) that he was not 'ranting and raving'. The veteran Conservative MP Sir Nicholas Soames, who just happens to be Winston Churchill's grandson, commented on Twitter: 'The President in full rant tonight. It seems he's acting heedless of grown up advice ... God knows what will happen with the big stuff.'
Which, of course, is why this is all so serious. In Moscow, Tehran and Pyongyang, calculations are being made: how can we test this most unpredictable and unhinged of US presidents? Long-range missiles are being test fired, a Russian spy ship is spotted 30 miles off the coast of Connecticut, and in the Black Sea, Russian fighter jets are photographed buzzing a US warship.
At his joint press conference with the Israeli prime minister, President Trump off-handedly ripped up one of the US's most hallowed foreign policy principles: that the only solution to the conflict between Israelis and Palestinians is to create an independent Palestinian state alongside the Jewish state.
These were his exact words: 'I'm looking at two-state and one-state and I like the one that both parties like. I'm very happy with the one that both parties like. I can live with either one. I thought for a while the two-state looked like it may be the easier of the two but honestly, if Bibi [Netanyahu] and if the Palestinians, if Israel and the Palestinians are happy, I'm happy with the one they like the best.' In other words, 'one-state, two-state, what do I care?'
It so happens that I agree with the president that the so-called two-state solution is no longer feasible. As I wrote in my recently-published memoir, after fifty years of illegal Israeli settlement-building on occupied Palestinian territory 'any attempt to create an independent Palestinian state would end up looking not so much like a patchwork quilt as like a succession of ink blots left behind by a careless colonial conqueror.'
But I wish I felt that Mr Trump had any idea what he was talking about. And I wish I didn't have a deep nagging fear that his reckless insouciance may well lead the Palestinians to conclude that the man in the White House needs to be taught a lesson about the reality of the conflict.
Last November, just a few days after his election victory, I wrote: 'The election of Donald Trump has made the world a much more dangerous place ... What scares me most about [him] is not only that he is a deeply unpleasant man with deeply unpleasant views but also that he is grotesquely, frighteningly incompetent and woefully unprepared for the task ahead ... For the next four years, the world will scarcely dare to breathe as we learn to live with a dangerous and unpredictable president in the White House.'
Four years? I'm not sure we can survive four years. Members of the US Congress now have a heavy responsibility resting on their shoulders; let's hope they understand where their duty lies.
Their duty to their country, and to the rest of the world. To rescue all of us before the 45th President of the United States of America has a chance to do any more harm.