There is an interesting problem connected with the practical significance of slips of the pen. You may recall the case of the murderer H. who managed, by asserting himself to be a bacteriologist, to obtain cultures of highly dangerous disease-germs from scientific institutions, but used them for the purpose of doing away in this most modern fashion with people connected with him. This man once complained to the authorities of one of these institutions about the ineffectiveness of the cultures sent him, but committed a slip of the pen and, instead of the words “in my experiments on mice and guinea-pigs (_Mäusen und Meerschweinchen_)”, the words “in my experiments on people (_Menschen_)” were plainly legible. This slip even attracted the attention of the doctors at the institute but, so far as I know, they drew no conclusion from it. Now, what do you think? Would it not have been better if the doctors had taken the slip of the pen as a confession and started an investigation so that the murderer’s proceedings might have been arrested in time? In this case, does not ignorance of our conception of errors result in neglect which, in actuality, may be very important? Well, I know that such a slip of the pen would certainly rouse great suspicion in me; but there is an important objection against regarding it as a confession. The matter is not so simple. The slip of the pen is certainly an indication but, alone, it would not have justified an enquiry. It does indeed betray that the man is occupied with the thought of infecting human beings; but it does not show with certainty whether this thought is a definite plan to do harm or a mere phantasy of no practical importance. It is even possible that a person making such a slip will deny, with the soundest subjective justification, the existence of such a phantasy in himself, and will reject the idea as a thing utterly alien to him. Later, when we come to consider the difference between psychical reality and material reality you will be better able to appreciate these possibilities. But this again is a case in which an error was found subsequently to have unsuspected significance.