 Premise (a) is seen as a platitude about memory (though some deny this; see section  3.3).  Premise (b) is regarded as stipulative, and premise (c) is thought to be the lesson of the slow switch experiment. The thinking behind (c) is this: Even if Oscar knows his water thoughts at t1, yet these morph into twin water thoughts at t2, he no longer knows at t2 about any water thoughts. For Oscar no longer has water thoughts at t2.