In "The Matrix and Philosophy" it explains about two main ways in which to distinguish metaphysically between real and unreal categories. The first one is source of the real and unreal, the second one is ontological status. In the dictionary source means the place where something begins, where it springs into begins and ontological is the metaphysical study of nature of being and existence. My own meaning of source is how everything began and where did it start from and ontological is how something depends on something else for its existence. I believe that Gracia and Sanford explained source . We don't know the source of all the things in the world of the matrix, but that's it's a very complex computer program made by arificially machine. That is how they defined source knowing that the matrix was made up by machine. The machine help built it.