“Some birds are not white” says there is at least one bird outside the class of white things. It does not tell us whether any white thing lies outside the class of birds. In a hypothetical domain every white thing could be a bird. So swapping terms alone does not yield a valid inference.
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Why particular negatives do not generally convert
Simple Conversion of Propositions: Definition, Method and Rules
“Some birds are not white” says there is at least one bird outside the class of white things. It does not tell us whether any white thing lies outside the class of birds. In a hypothetical domain every white thing could be a bird. So swapping terms alone does not yield a valid inference.