This quote means that if you don’t think something is possible, then you won’t be able to find it. It is similar to the saying “you have to believe it to see it.” This phrase implies that if you don’t believe in something, for example, magic, then you will never be able to have the experiences that come with it. In other words, if you don’t maintain an open-mind and look for possibility and potential, then you won’t find something until you start to believe in it.