 Certain cases put pressure on this. For instance, consider the famous Müller-Lyer illusion in which two lines of equal length look unequal. You can experience this even if you know (and therefore believe) that the lines are the same length. If perception were simply the acquisition of belief, then this would be a case of explicitly contradictory beliefs: you believe that the lines are the same length and that they are different lengths. But this is surely not the right way to describe this situation. (Armstrong recognized this, and re-described perception as a “potential belief”; this marks a significant retreat from the original claim).