He calls grinding an “illness” and offers various possible causes, including having an “inverted womb” (a variant on the clitoral hypertrophy motif), or a mis-match between the shape of a woman’s vagina and the shape of the penis. Another possible cause he lists is the mother’s diet during breast-feeding. Some women pick up the habit from having sex with concubines. Another explanation is that grinding is a natural appetite, derived from a variant of humoral theory.