"Who knows where it stops?"

Question

"There seem to be two very different kinds of things going on in the world: the things that be-long to physical reality, which many different people can observe from the outside, and those other things that belong to mental reality, which each of us experiences from the inside in his own case. This isn't true only of human beings: dogs and cats and horses and birds seem to be conscious, and fish and ants and beetles probably are too. Who knows where it stops?"

[What does it all mean? Thomas Nagel, chapter IV]

What does the author mean when he says "Who knows where it stops?"

Many thanks

Answer 1

The text says that humans are conscious, that some other animals seem to be conscious and that some smaller creatures probably are conscious. The final sentence "Who knows where it stops?"" appears to me to be to be asking two things. Firstly whether those listed smaller creatures probably conscious? And, secondly whether other smaller creatures and even other things, perhaps plants or fungi or bacteria, can be conscious? It might also be asking whether inanimate objects such as rocks or mountains or planets can be conscious.