The (unfortunate) consequences of this position for biology and evolution are now the subject of vigorous discussion at the blog Uncommon Descent, first here and now here. Barr wants to claim that scientific theories cannot confirm or deny the existence of purpose, and that any scientific theories (including materialism) which e.g. appear to deny purpose have to be reinterpreted. The trouble, then, is that the materialists would not even recognize their own theories any more.
In theistic science, by contrast, we explicitly make purpose an integral part of the operation of the universe. A universe, that is, which is not just material, but has also mental, spiritual and divine operations. According to theistic science, the grand claims of materialism are simply false. And hence also the exclusivity of natural selection in the production of biological creatures by means of (non-Darwinian) evolution. Only then can we have a proper causal account of how we have evolved. Being a causal account, we go further than activities investigating intelligent design, which deliberately refrain from examining the causes of design.