You hear it every time the discussion turns to immigration. How could it not, these days, given the sad images from the complicated crisis unfolding at our southern border?

But even before thousands of undocumented mothers and children from Central America were massing on our borders, immigration was a vexing issue.

It is particularly interesting when someone named Murphy or O’Brien or Callaghan -- or Hannity or O’Reilly -- rails against immigration, saying we should seal the borders, build a wall, whatever.

Inevitably, I will point out that their parents or grandparents or great-grandparents also came over.

“But they came over legally and they came here to work and they did everything they could to assimilate!” Murphy or O’Brien or Callaghan will confidently shout.

I will try to engage this person. I will note that plenty of Irish immigrants came over or stayed illegally and that Know Nothing nativists viewed the Irish as lazy sub-humans who were going to bankrupt America, spiritually and financially. Devoted Catholics could never assimilate, it was believed.

Alas, these nativist Hibernians generally don’t see the connection between then and now.

Such selective immigration memory is most popular among conservatives. But even passionate supporters of immigration, on the left, suffer from a variation of this historical amnesia.

Supporters on the left shout that 21st century immigrants deserve dignity. They point out that immigration is generally an economic boost to the country, and they dismiss anti-immigrant activists as bigots.

But here’s what they rarely do: use America’s past experiences with immigration to illuminate the future of immigration.

Consider a new book by Aviva Chomsky entitled Undocumented: How Immigration Became Illegal. Chomsky is the daughter of famous radical writer Noam Chomsky, so her progressive credentials are solid. As you can imagine, she makes a strong (if unrealistic) case for more open borders and more dignified and humane treatment of immigrants.

But while making her academically fashionable argument for how immigration “became” illegal (just as Noel Ignatiev once argued that the Irish “became” white), readers could be excused if they believed illegal immigration only “became” a problem around 2001.

Chomsky barely mentions the Irish immigrant experience in America. Love immigration or hate it, people on both sides of the issue could probably agree on one thing: the past can show us what the future might hold.

It would seem sensible for Chomsky – in a book about the history of “undocumented” immigrants – to explore immigrants from Ireland who snuck in through Canada, or railroad laborers over for seasonal work who decided they liked America enough to stay.

But no. Instead, Chomsky and others focus solely on today’s immigration, which is not only shortsighted, it does nothing to actually engage today’s opponents of immigration. It actually plays right into their misguided notion that immigration in the old days must have been just dandy.

Worse, if they ever do acknowledge past immigration, critics like Chomsky insist that today’s immigrants face a far more hostile, racist society because past immigrants “decided” to “become” white.

Maybe. Maybe not. An equally strong argument could be made that between the terrible hygiene, lack of social safety net and pervasive religious hostility, 19th century Irish immigrants faced more dire conditions.

But that’s not the point. Immigrants from both eras faced grave hardship. But if we acknowledge that America is now stronger because of 19th century immigration, doesn’t that make a compelling case for 21st century immigration?

If today’s immigration advocates truly want justice and dignity for today’s immigrants, they need to remind today’s nativists that their grandparents and grand-parents were not always “documented.”

Doing that, however, would require them to acknowledge 19th century immigration. Until they do that, those who claim to be defending immigrants are proving themselves to be nearly as shortsighted as anti-immigrant Irish Americans.

(Contact “Sidewalks” at tdeignan.blogspot.com)