Our perceptual experiences depend on stimuli which arrive at our various sensory organs from the external world and these stimuli cause changes in our mental states , ultimately causing us to feel a sensation , which may be pleasant or unpleasant . Human perceptual experiences depend on stimuli which arrive at one 's various sensory organs from the external world and these stimuli cause changes in one 's mental state , ultimately causing one to feel a sensation , which may be pleasant or unpleasant .