Biologist Richard Dawkins makes a case for "thinking the improbable" by looking at how our human frame of reference -- the things we can perceive with our five senses, and understand with our eight-pound brain -- limits our understanding of the universe. Think of it: We can't see atoms, we can't see infrared light, we can't hear ultrasonic frequencies, but we know without a doubt that they exist. What else is out there that we can't yet perceive -- what dimensions of space, what aspects of time, what forms of life? Dawkins calls the human-size frame of reference "Middle World": between the microcosmos of atoms and the macrocosmos of the universe. Middle World thinking limits our ability to see the universe in terms of the improbable, whereas "in the vastness of astronomical space and geological time, that which seems impossible in Middle World might turn out to be inevitable."