Feb. 17 -- 1961
Dear Professor Lederberg,
I am teaching this year at Yale.  Your letter has been sent to this address.  I have tested the system in many lectures, I asked the Auditors to guess what certain unknown symbols meant.  People on a scientific level had no difficulties.  I learned some improvements from individual mistakes.  It is surprising that less educated people or even scholars in the humanities have the greatest trouble to [ . . . ] things.
I do not know a better way to introduce variables.  Do not confuse unknowns (indicated by ?x . . . ) with variables.  What you represent are, to my opinion, unknowns, which have one value only (though unknown) not variables ranging over a certain set.  If, on the other hand, you mean to write 1 + a = 2 --> a =1, when the --> is omitted, you settle a syntax's which I have never admitted throughout my exposition, viz. to omit connectives thus are the most essential part of the communication.
I thank you much for your interest and your bibliographical remark.
Sincerely yours,
Hans Freudenthal