Two great themes emerge from Barack Obama's most important presidential speeches.

The first is of America as an ever unfinished work in progress, a place where we draw strength from our ability to forever make our union more perfect. "What greater expression of faith in the American experiment [is there] than this?" he said in his speech at the 50th anniversary of the Bloody Sunday march in Selma, perhaps the most important of his presidency. "What greater form of patriotism is there than the belief that America is not yet finished, that we are strong enough to be self-critical, that each successive generation can look upon our imperfections and decide that it is in our power to remake this nation to more closely align with our highest ideals?"

And then there's the second theme. Like any president, Obama is required to speak on sad, mournful occasions as well as celebrations and anniversaries. We ask the president to bind up the nation's wounds, to bring us all together, to make meaning out of tragedy, and create hope with his eloquence. Obama has had to give that kind of speech again and again, and Tuesday he gave another, in a ceremony in Dallas honoring the five officers killed there last week. In speeches like this one, he's more likely to return to his other great theme, that not only are we less divided than we might appear, but that our divisions can be bridged if only we allow ourselves to understand one another.

This was the theme of the speech Obama gave at the Democratic convention in 2004 — the one that, it is no exaggeration to say, enabled him to become president four years later. Even then he offered himself as a translator, one who could stand between opposing factions and explain to each what the other side felt and believed. And out of that act of translation, he argued, understanding and eventually unity could emerge.

As his presidency winds down, it's almost painful to recall the hope of healing he once embodied — and how brutally those hopes were dashed. This president, who has tread so carefully over so many controversies, was met by his political opponents with not just disagreement but with a boiling hatred and the vilest calumnies — that he isn't an American, that he hates cops, that he hates white people, that he is literally trying to destroy the country. But he never stops insisting on the possibility of unity, and he did so again in Dallas. Here's part of what he said:

Today, in this audience, I see people who have protested on behalf of criminal justice reform grieving [alongside] police officers. I see people who mourn for the five officers we lost, but also weep for the families of Alton Sterling and Philando Castile. In this audience, I see what's possible. I see what's possible when we recognize that we are one American family, all deserving of equal treatment. All deserving equal respect. All children of God. That's the America I know. [Obama]

Yet watching the speech, I couldn't help but be gripped by sadness, not just for the slain officers and their loved ones, but for the dream Obama still insists can become real. While the president was preparing to deliver his speech, the vulgar buffoon his political opponents have chosen to replace him, who has not only run a campaign of bigotry and xenophobia but who insists that our divisions are all but unbridgeable, tweeted, "President Obama thinks the nation is not as divided as people think. He is living in a world of the make believe!"

Maybe so — Donald Trump is certainly doing everything he can to keep the country as divided as possible. But at times in his speech Tuesday, Obama had an almost pleading tone in his voice, imploring us not to give up on the idea of common American identity and purpose.

Obama's Dallas speech serves as a bookend to the convention speech he gave in 2004. Back then he offered his personal story as evidence of the ties binding this country together. He himself was the living proof of his words, the embodiment and synthesis of our complexity. "I stand here knowing that my story is part of the larger American story, that I owe a debt to all of those who came before me, and that in no other country on Earth is my story even possible," he said. "Alongside our famous individualism, there's another ingredient in the American saga, a belief that we are all connected as one people," he said. And most memorably: "There's not a liberal America and a conservative America; there's the United States of America. There's not a black America and white America and Latino America and Asian America; there's the United States of America."

Those were the words of a younger, more idealistic man who had yet to accumulate the gray hair and battle scars of the president we see today, a man who was still being praised by his political opponents, a man who hadn't spent eight years fending off attacks from all directions. Obama may still believe, but his words on Tuesday were far heavier and more fraught. He talked about the prejudices none of us want to admit, the crippling burden discrimination imposes on people's everyday lives, the failures of policy that mean "we ask the police to do too much, and we ask too little of ourselves."

"Can we do this?" he asked in Dallas. "Can we find the character as Americans to open our hearts to each other? Can we see in each other a common humanity and a shared dignity and recognize how our different experiences have shaped us? And it doesn't make anybody perfectly good or perfectly bad, it just makes us human. I don't know. I confess that sometimes I, too, experience doubt. I've been to too many of these things. I've seen too many families go through this."

Obama ended on an optimistic note, as he must. "With an open heart," he said, "we can learn to stand in each other's shoes and look at the world through each other's eyes." It's something we can all hope for. I just wish I could believe it will happen any time soon.