 Notable as well are the lives of Amadou Bamba and Hajj Umar Tall in sub-Saharan Africa, and Sheikh Mansur Ushurma and Imam Shamil in the Caucasus region.In the 20th century some more modernist Muslims have called Sufism a superstitious religion that holds back Islamic achievement in the fields of science and technology.A number of western converts to Islam have also embraced Sufism, sometimes resulting in considerable syncretism or generic spiritualism detached from Islam, as in the case of "Universal Sufism" or the writings of René Guénon or G. I. Gurdjieff.