Now the danger we are in when we adopt the sense datum notation is to forget the difference between the grammar of a statement about sense data and the grammar of an outwardly similar statement about physical objects. (From this point one might go on talking about the misunderstandings which find their expression in such sentences as: “We can never see an accurate circle”, “All our sense data are vague”. Also, this leads to the comparison of the grammar of “position”, “motion”, and “size” in Euclidian and in visual space. There
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is, e.g., absolute position, absolute motion and size in visual space.)