"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?"