 The arguments of DKB8/LMD8 demonstrate how what-is must be. In applying these arguments as tests against any suggested basic entity in the Presocratic search for ultimate causes or principles, the kouros can determine whether or not a proposed theory is acceptable. For Parmenides noos is not itself an infallible capacity. One can think well or badly; correct thinking is that which takes the correct route and so reaches what-is. The mortals on the incorrect route are thinking, but their thoughts have no real object (none that is real in the appropriate way), and so cannot be completed or perfected by reaching the truth. In B8 Parmenides sets out the criteria for the being of what-is, and then the arguments for those criteria: