 The fallacy of accident is the most elusive of the fallacies on Aristotle’s list. It turns on his distinction between two kinds of predication, unique properties and accidents (Top. I 5). The fallacy is defined as occurring when “it is claimed that some attribute belongs similarly to the thing and to its accident” (SR 5 166b28). What belongs to a thing are its unique properties which are counterpredicable (Smith 1997, 60), i.e., if \(A\) is an attribute of \(B\), \(B\) is an attribute of \(A\). However, attributes that are accidents are not counterpredicates and to treat them as such is false reasoning, and can lead to paradoxical results; for example, if it is a property of triangles that they are equal to two right angles, and a triangle is accidentally a first principle, it does not follow that all first principles have two right angles (see Schreiber 2001, ch. 7).