Full Idea
If one tries to observe oneself in the present one finds in oneself only the state of observing oneself. Introspection, then, can only work in the case of past states of mind, …about which one can always be mistaken.
Gist of Idea
Observing oneself in the present is impossible, and oneself in the past may be wrong
Source
Simone Weil (Lect 1: Materialist Viewpoint [1933], p.28)
A Reaction
I don't think this is wholly correct. We can become self-aware for a moment while absorbed in something else, and have some insight into what the absorption was like. Some aspects of past experience can be known accurately.
Book Reference
Weil,Simone: 'Lectures on Philosophy' [CUP 1978], p.28