I point out that Mack and Rock manipulated both expectation and attention and suggest that their results may have been caused by lack of expectation rather than lack of attention. This alternative reading of Mack and Rock's results is supported by other findings, which suggest that 'pure' manipulations of expectation produce 'blindness' whereas 'pure' manipulations of attention do not. Why should failure to expect or anticipate a stimulus lead to 'blindness'? In psychophysics, stimuli near threshold typically require a degree of (...) familiarity to be consciously perceived. Perhaps the same is true for the supra-threshold stimuli used by Mack and Rock. This may reflect the fact that the human visual system uses natural and acquired 'priors' to solve the probabilistic problem of perception. (shrink)