A machine may be consistent despite lack of proof
To defeat Newman and Nagel's thesis, Putnam must show that the consistency of the machine is absolutely undecidable. But there are no absolutely undecidable propositions in arithmetic.
The best Putnam can offer is the unlikelihood of being able to show that the machine in question is inconsistent.
But the fact that it is unlikely that we can show that a machine is consistent doesn't mean that the machine is in fact inconsistent.
Thomas Tymoczko (1990).