On reflecting on all you have said in this thread here, I have come to see that there is an argument to settle this issue once and for all. It goes like this:

1: If there is no way to distinguish true thoughts from false ones, all thoughts become meaningless.

2: Not all thoughts are meaningless.

3: There is a way to distinguish true thoughts from false ones (from 1 and 2.)

4: The only way to distinguish true thoughts from all others is if there is a correspondence between the thought and the reality.

5: There is a correspondence between true thoughts and reality (from 3 and 4.)

6. Nothing immaterial corresponds to reality.

7. Anything not immaterial is material (from the law of the excluded middle.)

Conclusion: True thoughts must be material (from 5, 6 and 7.)

- So there you have it!