Simplicity refers to the number of `auxiliary hypotheses' a theory makes to explain a phenomenon. Auxiliary hypotheses presuppose properties or relations between observable or postulated entities in the natural process in order to explain or predict a subset of the phenomena within the consilience of the theory. Thagard [Thagard, 1988] gives the example of Huygens' theory of light. In this theory it is assumed that waves are spherical. However, in order to explain the irregular refraction from Iceland crystal, Huygens assumed a property of the postulated entity `wave': ``some waves are spheroidal''. This extra assumption was introduced to deal with the problematic Iceland crystal case, but reduced the simplicity of the theory. The theory with the minimal number of auxiliary hypotheses is generally preferred.