Barack Obama has used his first high-profile speech since stepping down as US President to say "you have to believe in facts".
His comments are seen as a riposte to the current US administration's use of "alternative facts".
"Without facts there is no basis for co-operation," he said at the annual Nelson Mandela lecture in South Africa.
"If I say this is a podium and you say this is an elephant, it is going to be hard for us to co-operate."
Obama said that he could find common ground with people who disagree with the Paris accord on climate change - which Donald Trump wants to pull the US out of - if they have an argument based on fact.
But, he added: "I can't find common ground if someone says climate change is not happening when almost all the world's scientists say it is. If you start saying it is an elaborate hoax, where do we start?"
A moment later, Obama told the audience of 15,000 people: "It used to be if you caught them [politicians] lying, they said: 'Oh man'. Now they just keep on lying."
His speech is part of events to mark 100 years since the birth of former South Africa President Nelson Mandela, who died in 2013 aged 95.
Both men were the first black presidents of their countries.
Speaking on Tuesday in South Africa's main city, Johannesburg, Obama also said politicians using "politics of fear, resentment, retrenchment" were rising "at a pace unimaginable just a few years ago".
Barack Obama offered what some of his supporters will see as not-so-veiled references to his successor in his speech honouring Nelson Mandela's legacy, defending democratic institutions and a free press, and condemning "strongman politics" and shameless leaders who "double down" when caught in lies.
The former president also offered a commodity he always seems to have in ready supply - hope.
"Things may go backwards for a while, but ultimately, right makes might," Obama said. "Not the other way around."
It's a riff on the Theodore Parker line he frequently quotes, about the arc of history being long but bending toward justice.
If Obama had a message for the world - and particularly for Americans unsure about the course their nation is on - it's that the struggle is real, but the ending is a happy one.
There are probably more than a few on the left, however, who wish Mr Obama would give more than a few speeches and carefully worded statements.
With mid-term elections that will determine control of Congress just four months away, they want him to step away from the podium and fully join the fight.