When a person believes that there is nothing beyond what they currently experience, the mind reaches a state of perceptual stagnation.
Very few people think there is nothing beyond what they
currently experience, because we experience very little of the whole world. Think about astronomers. They might believe there is nothing beyond what could
possibly be experienced, yet they don't lack any imagination to create new ways of seeing beyond what could previously be seen and thereby discover new worlds. And sometimes, these new words blow our minds: there are many quite amazing stuff in the universe.
So believing there is nothing beyond what can be experienced does not produce stagnation of the mind, on the contrary: it gives an incentive to expand our ability to experience things.
A common example of this most people might recognize is reading a book verses watching the same story in movie form. When one watches a movie, one is fed the sounds, pictures, and scenarios directly and almost completely based on ones senses, rather than ones thought process. When a person reads a book it has been proven that the mind is more active because it in fact must create all of the previously mentioned sensory images from ones mind, either imagined or experienced in the past. If this part of the mind is not exercised, as any muscle left inactive, its ability to function will atrophy just like a muscle. Occasionally the mind will be called upon to invoke images for which it has no previous association. This is when the mind is truly exercised and has potential to create something new and amazing.
Yes, imagination is great. And fantasy is often an effective way to exercise it. But you don't need to actually believe it. I like Star Wars movies and yet I don't believe in the power of the Force.