According to critics, psi is negatively defined as any effect that cannot be currently explained in terms of chance or normal causes and this is a fallacy as it encourages parapsychologists into using any peculiarity in the data as a characteristic of psi. Parapsychologists have admitted it is impossible to eliminate the possibility of non-paranormal causes in their experiments. There is no independent method to indicate the presence or absence of psi. Persi Diaconis has written that the controls in parapsychological experiments are often loose with possibilities of subject cheating and unconscious sensory cues.