 Perhaps the most widely accepted view along these lines is that self-knowledge, even if not absolutely certain, is more secure, epistemically, than other kinds of empirical knowledge—most obviously, perceptual knowledge. Some who take this line maintain that there is a causal gap between a perceptual state and its object, and this gap introduces sources of error that are absent in direct introspective apprehension of a sensation (Russell 1917; Chalmers 2003; Gertler 2012; Horgan 2012; Siewert 2012).