Parapsychology has been a controversial science since its inception, with experimenters claiming results that cannot be explained by chance and critics arguing that the best explanations are methodological flaws, statistical errors, or outright fraud. The origins of parapsychology are often traced back to the start of spiritualism, a Christian faith with the added dimension of spirit communication through mediumship. Most of the standard methods of contemporary experimental research in parapsychology use unselected subjects. The most important of these are the free-response extrasensory perception (ESP) techniques and psychokinesis (PK) with random number generators. One of the best known attempts was the ESP-dream research begun by Montague Ullman in 1960. PK research has come a long way since the studies of table-tipping and levitation. In 1970, Helmut Schmidt, at the Durham Institute for Parapsychology, began work with a new kind of PK machine.