 Boethius’s second type of equivocation is ‘analogia’ in the Greek sense, and the standard example was the word ‘principium’ (principle or origin), which was said to apply to unity with respect to number and to point with respect to a line, or to both the source of a river and the heart of an animal. ‘Principium’ is a noun and, as such, might be expected to pick out a common nature, but although a unity, a point, a source and a heart can all be called ‘principium’ with equal propriety, there is no common nature involved. Mathematical objects, rivers, and hearts, represent not merely different natural kinds, but different categories, in that mathematical objects fall under the category of quantity, and hearts at least under the category of substance. What allows these disparate things to be grouped together is a similarity of relations: a source is to a river as a heart is to an animal — or so it was claimed. While theologians, including Aquinas himself in De veritate, and the fourteenth-century Dominican Thomas Sutton, occasionally make use of this type of analogy, most logicians do not even mention it.