But there is a really big difference in believing in a metaphysical world as an adult, and believing in the tooth fairy as an adult. You can actually prove or disprove he existence of the tooth fairy as we describe it to be, to be real or not wih enough testing. If 1,000,000 different people everyday leave their tooth under a pillow, a fairy with the characteristics of the classical western "tooth fairy" will enter your room and switch the tooth with money. That is a scientific hypothesis. You can monitor the room, and see that it never happens after millions of trials. And like all scientific fact, it will 99.9999% prove that the tooth fairy does not exist. The existence of metaphysics in general, is not. If I pray 100 times a day, I will go to heaven. This is not a scientific hypothesis. You cannot test that.