 Further, it is not obvious that all a priori knowledge rests on grounded concepts. Normative or mathematical concepts might map the normative and mathematical domains but not the external world. We can know a priori that it is wrong to torture children just for the fun of it and that two is the only even prime regardless of what the external world is like. Perhaps we can also know a priori that some general normative principles are true, such as the principle of inference to the best explanation (IBE). Roughly, this principle says that we are justified in believing some hypothesis if it is the best explanation of what we observe. For instance, it says that we are justified in believing that someone recently walked along the beach because that best explains our observation of footprints in the sand. But we could not be justified in accepting IBE because it is useful and the best explanation of its usefulness is that it fits the way the world is. That would be a circular argument for accepting IBE. Lastly, it seems possible for even a BIV to know certain conditional propositions, for instance, to know that that IF something is a vixen, it is a female fox and IF someone is bachelor, he is an unmarried male.