Our planet is finite. There is a finite surface, finite resources, and even finite time.
On a planet without humans, there would be as little as 25 million years remaining for any complex life.
It took until around the 1700s for many to know the finite surface, and until the 20th century for many to consider implications of finite resources and think sustainability. One finite planet is about the next step: recognising that time is also finite and, even provided humanity does nothing to end things prematurely, there is as little as 2% of the time Earth can support now remaining. Sustainability alone, is a death sentence.