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Originally Posted by MadLuke
The statement cannot be true,

Check the truth table for logical implication "P implies Q". Also, construct the equivalent table for " 'not P' or Q".
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because if Tretiak is the greatest goaltender it is impossible for other goaltender to be.

Yes, but if he's not (i.e. the premise is false), then the implication is automatically true.
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Even if tretiak is not the greatest, if he is Roy cannot be.

You just posted a contradiction ("Tretiak is not the greatest" and "he is the greatest").