On the contrary, it is a matter of non-arguable and generally accepted proof. Until that day, encounters of extraterrestrial or religious kind will be claimed to be personal privileges. As we witness throughout our lives, persons have individual minds that can play tricks to their owners, lie, or simply believe. Therefore, we require proof when we are socially urged to believe the existence of something. For example, nobody questions the life-saving, or at least `life-repairing`, function of the medical science. Although it does not offer 100% success rate, it is universally accepted as the best option. It is because we all, to a varying degree, witnessed its efficiency. On the other hand, you cannot convince a cancer patient about the existence of the cure for cancer until that person is cured.