When observing the universe, we realize it can sustain life—if it couldn’t, we wouldn’t be here. This notion has been articulated numerous times throughout history, but it lies at the core of the philosophical debate known as the principle of humanity. While seemingly straightforward, it holds complexities.

The universe seems to be delicately balanced on the edge of habitability, which leads to what scientists call “tweaking problems.” Several fundamental constants, from the mass of neutrons to gravity, require precise values for life to exist. “If some of these constants were larger, it could destabilize all atoms,” says Luke Burns from Western Sydney University, Australia.

The principles of humanity originated as a way to explain why the universe appears to be in this seemingly favorable condition, distilled into a simple idea: the universe must be this way, or we wouldn’t be here to observe it.

There are two central formulations of this principle, both articulated in a 1986 book by cosmologists John Barrow and Frank Tippler. The weak principle states that the basic constants of the universe must be in a state compatible with the existence of life—at least here and now. The strong principle goes further, asserting that these constants must lie within a range conducive to life, implying that the universe is designed to support it. This notion of “necessity” indicates that the universe exists to foster life.

If the weak principle suggests, “A tree falls in the forest and life must be able to thrive there,” the strong principle posits, “This planet is destined to have a forest where the tree can flourish.”

For contemporary scientists, the weak principle acts as a reminder of potential biases in our observations of the universe, especially if conditions are not uniform everywhere. “If we lived in a universe different from our current one, we’d find ourselves in conditions where life was feasible,” notes Shawn Carroll from Johns Hopkins University in Maryland.

As for the strong formulation of the principle, some physicists, like Burns, find it useful. He is exploring various multiverse models and sees the strong principle as a practical benchmark. This implies there’s a 100% chance at least one life-supporting universe will arise within the multiverse framework. Therefore, the closer a multiverse model approaches this 100% likelihood, the more plausible it becomes. Conversely, if the probability is around 50%, he views it as a solid signal of the model’s validity. “But if it hits a square meter, we have a problem,” he states.

Despite its utility, most physicists regard the strong principle as overly deterministic. It implies life was always meant to be present, according to Elliot Thorber from the University of Wisconsin-Madison. “However, the likelihood is minimal; life could have failed to emerge, and we would still be making the same observations.”

Where does that leave us? The strong extrinsic principle offers a solution to the fine-tuning dilemma, yet many consider it an irrational conclusion. In contrast, the weak principle doesn’t clarify why our universe’s constants are finely tuned, though it remains a valuable analytical tool for researchers. As principles go, this topic is quite complex.