Dear Nikola,

Thank you for your letter, and please forgive my immediate reply. Obviously a reply may be delayed, from inattentiveness or indifference, for too long.  A reply may also be too soon, immediate and overly precipitous, indicating that the respondent is concerned primarily with expounding his own ideas, and is insufficiently sensitive, or is indifferent to the experienecs to to which he is replying. Such indeed is a fault to which I should often plead guilty.  I excuse myself by quoting the Bible,
Wes das Herz voll ist /des gehet der Mund ueber. (Luther's translation)
For out of the abundance of the heart the mouth speaketh. (Matthew 12:34)
ex abundantia enim cordis os loquitur (Vulgate)
ἐκ γὰρ τοῦ περισσεύματος τῆς καρδίας τὸ στόμα λαλεῖ. 

To attempt to come to terms with my ignorance, I try to understand my ignorance, which I interpret as largely the deficiency of memory.  Whom may I blame for the deficiencies of my memory?  As I read the sentences, the paragraphs and the pages of Feynman's lectures over and over again I think I understand each single argument; but then, when he strings them together I can't remember the meanings of the conclusions that he has proved. Then I fall asleep and awaken to the nightmare that I can't possibly pass the examination and that there's no way of denying the fact that am failing, that I have failed the course, - which is what being 90 years old is all about.

Good night. I hope you sleep well, with edifying dreams.
EJM