``There is more to seeing than meets the eyeball''
N R Hanson (1958)
What an observer sees, that is, the visual experience that an observer has when viewing an object depends in part on past experience, knowledge and expectations. Individual experiences cannot be logically deduced to be identical or even the same.
Example:
Changes in medical students perceptual experience when being taught to make a diagnosis by inspecting an X-ray film
``Think of a medical student attending a course in the X-ray diagnosis of pulmonary diseases. He watches, in a darkened room, shadowy traces on a fluorescent screen placed against a patient's chest, and hears the radiologists commenting to his assistants, in technical language, on the significant features of these shadows. At first, the student is completely puzzled. For he can see in the X-ray picture of a chest only the shadows of the heart and ribs, with a few spidery blotches between them. The experts seem to be romancing about figments of their imagination; he can see nothing that they are talking about. Then, as he goes on listening for a few weeks, looking at ever-new pictures of different cases, a tentative understanding will dawn upon him; he will gradually forget about the ribs and begin to see the lungs. And eventually, if he perseveres intelligently, a rich panorama of significant details will be revealed to him: of physiological variations and pathological changes, of scars, of chronic infections and signs of acute disease. he has entered a new world. He still sees only a fraction of what the experts can see, but the pictures are definitely making sense now and so do most of the comments made on them.''
M Polanyi (1973)