Human solipsists have navels. There is no doubt about that. As CarolC has cleverly inferred, this confronts the solipsist with direct evidence of the fact that he once had a mother...and by implication a father. This despite the fact that his view of reality is that only the self (his own self) can be known to exist! (Or her own self, in the case of a female solipsist.) The navel is a direct refutation of that false presumption.

The way the solipsist gets around that is by misinterpreting the evidence provided by the navel or simply ignoring it. This is similar to the way people in religions willfully misinterpret or ignore various information to suit their basic assumptions about reality. It is also true of many atheists and non-religious people, although their use of the same deceptive strategem is usually more subtle than in the case of the religionist.

Chongo - Well, I am pleased to see that you are proud of your species, and not afraid to take a stand in its defence. I will readily agree that apes are far less destructive to the natural environment than humans, and that indicates that they are more in tune with Divine Plan in a number of ways than their human brothers and sisters are. However, don't forget that it took a human mind to erect the Empire State Building! King Kong merely climbed up to the top of it.