It is sometimes suggested that presence should be measured in terms of the degree to which a real scene is indistinguishable from a virtual scene, or in terms of the amount of noise needed to make a real scene indistinguishable from a virtual scene [62,97,91,94]. These might be called ``realism'' measures. Such measures are ``Class A'': they require only a comparison of sensory input, between the real and virtual environments. However, I do not believe that they are satisfactory. The reason is that realism does not appear to be the key factor determining presence. For instance, movies based on animation are no less engaging (or profitable) than those based on realistic imagery. Pausch et al., in summarizing Disney Imagineering's experience with a virtual environment based on the movie ``Aladdin'', mention the importance of content (not simply realism) to the sense of presence . Consistent with this, Welch et al.  have recently published results in which pictorial realism was less important to reported presence than either interactivity or short delays.