Comment by card_zero

Natural doesn't mean likely. Say life rarely gets started, because it requires some kind of accidental evolutionary engine involving rivers and clay crystals, some unusual conditions of weather and geology. Then say life rarely gets complex and big, because mats of bacteria can be the dominant species indefinitely. So the universe is mostly dead, and the living parts are mostly slime. Then say actual human-like intelligence, the kind that tries really hard to imagine new things to meddle with and new spaces to explore, such as exploring the space space, is a freakish mutation and is unlikely to be adaptive at first. So it rarely happens and then usually dies out straight away. The rare instances of complex life, then, are mostly just floating around in oceans wiggling their complex limbs fecklessly. So those are two terms in the Drake equation, with an extra one about complexity added in the middle, and they multiply together to make things very unlikely by an unknown amount. We don't know what the numbers are. It might be natural that there isn't any sign of life out there, if the small probability of spacefaring life is smaller than space is big.