I say, and with God's success: The impurity in the shoe may have a substance, or it may not have a substance: If it has a substance, it may be dry or wet: If it is dry, it is purified by rubbing; because this removes the trace of the impurity. If it is wet, it is purified by rubbing on the condition that one exaggerates in it so that no smell or color of the impurity remains on what is ruled upon. However, if the impurity does not have a substance, like urine, it does not leave a trace after drying, and it is not purified except by washing. See: Al-Wiqayah and its explanation by Sadr al-Shari'ah, p. 130, and Nafi' al-Mufti, p. 137, and God knows best.