1 daniel m. wegner , the illusion of conscious will ( bradford books , massachusetts institute of technology press , cambridge , 2002 ) ______________________________ chapter 7 : nobody knows gravity when scientists want to explain whether or not the theory of evolution is a fact , they often say that it is as much of a fact as gravity , which sounds convincing but might lead a person with a skeptical bent to wonder just how much of a fact gravity really is .