If humans have some inherent flaw which makes them blind to the existence of their tails, then they must also have an inherent flaw which makes them blind to mass measurements that include the tail, and blood flow measurements that include the tail, and the source of pain being the tail when the tail is cut off, and the fact that they have designed pants with holes in them to fit the tail, etc. You end up postulating a whole series of inherent flaws permeating our ability to do very basic things. That's plausible for stroke victims, but not really for humanity in general unless the entirety of modern medicine and science is fatally flawed.