a radical hypothesis such as an extraterrestrial origin of ufos requires overwhelming proof in order to be generally accepted.
ufo来自于地球外根本就是一个大胆的假设,接受这种假设需要有非常强说服力的证据。
typically, witnesses who take the trouble to report a sighting consider the object to be of extraterrestrial origin or possibly a military craft but certainly under intelligent control.