There is an interesting problem linked with the practical significance of the pen slip. You may recall the case of the murderer H., who made a practice of obtaining cultures of the most dangerous disease germs from scientific institutions, by pretending to be a bacteriologist, and who used these cultures to get his close relatives out of the way in this most modern fashion. This man once complained to the authorities of such an institution about the ineffectiveness of the culture which had been sent to him, but committed a pen slip and instead of the words, "in my attempts on mice and guinea pigs," was plainly written, "in my attempts on people."[20] This slip even attracted the attention of the doctors at the institution, but so far as I know, they drew no conclusion from it. Now what do you think? Might not the doctors better have accepted the slip as a confession and instituted an investigation through which the murderer's handiwork would have been blocked in time? In this case was not ignorance of our conception of errors to blame for an omission of practical importance? Well, I am inclined to think that such a slip would surely seem very suspicious to me, but a fact of great importance stands in the way of its utilization as a confession. The thing is not so simple. The pen slip is surely an indication, but by itself it would not have been sufficient to instigate an investigation. That the man is preoccupied with the thought of infecting human beings, the slip certainly does betray, but it does not make it possible to decide whether this thought has the value of a clear plan of injury or merely of a phantasy having no practical consequence. It is even possible that the person who made such a slip will deny this phantasy with the best subjective justification and will reject it as something entirely alien to him. Later, when we give our attention to the difference between psychic and material reality, you will understand these possibilities even better. Yet this is again a case in which an error later attained unsuspected significance.