Recently the philosopher William MacAskill, with his book What We Owe The Future, has been popularizing the idea that the fate of humanity should be our top moral priority. His core proposition is that today's 8 billion humans are vastly outweighed in importance by the hundreds of billions of humans who could live in future generations if we can avoid wiping out humanity in the near term. MacAskill's argument is known by the slogan "longtermism," (often written as long-termism) and it has already been sharply criticized. For example, columnist Christine Emba has written in The Washington Post: "It's compelling at first blush, but as a value system, its practical implications are worrisome." In practice, she explains, it implies seeing "preventing existential threats to humanity as the most valuable philanthropic cause"--which means we should invest far more in addressing risks that threaten humanity's very long-term existence. As Emba says, this can seem impossible to disagree with.

Although most cultures, particularly in the west, provide a great many commemorations of distant ancestors – statues, portraits, buildings – we are much less willing to consider our far-off descendants. We might invoke grandchildren, at a push great-grandchildren, but after that, it all becomes a bit vague and, well, unimaginable. And while we look with awe and fascination at the Egyptian pyramids, built 5,000 years ago, we seem incapable of thinking, or even contemplating, 5,000 years in the future. That lies in the realm of science fiction, which is tantamount to fantasy. But the chances are, barring a global catastrophe, humanity will still be very much around in 5,000 years, and going by the average existence of mammal species, should still be thriving in 500,000 years. If we play our cards right, we could even be here in 5m or 500m years, which means that there may be thousands or even millions times more human beings to come than have already existed.