We use the phrase “two books have the same colour”, but we could perfectly well say: “They can't have the same colour, because, after all, this book has i[y|t]s own colour, and the other book has its own colour too”. This also would be stating a grammatical rule, ‒ ‒ ‒ a rule not in accordance with our ordinary usage. The reason why one should think of these two different usages at all is this: We compare the case of sense data with that of physical bodies in which case we make a distinction between: “this is the same chair that I saw an hour ago” and “this is not the same chair, but one exactly
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like the other”. Here it makes sense to say, and it is an experiential proposition: “A and B couln't have seen the same chair, for A was in London and B in Cambridge; they saw two chairs exactly alike”. (Here it will be useful if you consider the different criteria for what we call the “identity of these objects”. How do we apply the statements: “This is the same day … ”, “This is the same word … ”; “This is the same occasion … ”, etc.?)