We say ‘I recall the subject matters x and y about which I discussed with the person z’ only if that person z has a low significance for us. If the person z has a great significance, we say ‘I recall that I discussed with z about the subject matters x and y’.
The difference tells much about the secondary value of speech. Those to whom we speak seem to be more important.
Nonetheless, we remember better the subject matters than the people we talked to in those areas where the man should occupy the first position, as it is the case of politics, ethics, religion, etc.
For instance, we always can remember an argument or a theory of freedom or morality and often we forget who the person we addressed to was. For in these areas it has instituted the practice of ignoring the interlocutors. They become only abstract interlocutors and are melted away in the abstractness of our discourses or conversations even if they are really present (we see them only as defenders of a certain position or thesis).
We may suspect that the persons are ignored exactly for concealing the low value of speech, since the speech seems to be the core of those areas and not the reality.