Full Idea
For Armstrong, introspection involves a belief, and mental states and their accompanying beliefs are 'distinct existences', so a state without belief shows states are not self-intimating, and the belief without the state shows beliefs aren't infallible.
Clarification
'Self-intimation' is a mental state being obvious to its possessor
Gist of Idea
A mental state without belief refutes self-intimation; a belief with no state refutes infallibility
Source
report of David M. Armstrong (A Materialist Theory of Mind (Rev) [1968]) by Sydney Shoemaker - Introspection
Book Reference
'A Companion to the Philosophy of Mind', ed/tr. Guttenplan,Samuel [Blackwell 1995], p.396
A Reaction
I agree with Armstrong. Introspection is a two-level activity, which animals probably can't do, and there is always the possibility of a mismatch between the two levels, so introspection is neither self-intimating nor infallibe (though incorrigible).