 The first three are the most often cited, the fourth is often not labeled, and the fifth has regularly been mislabeled as “it is a matter of indifference that”, and so likewise for the sixth. Typically, one of the first two is taken as primitive, and the others defined in terms of it, but any of the first four can play the same defining role. The most prevalent approach is to take \(\OB\) as primitive, and define the rest as  follows:[11]