There is still the additional difficulty that we frequently do not know the circumstance with the help of which the witness has made his association. Thomas Hobbes tells the story of an association which involved a leap from the British Civil War to the value of a denarius under the Emperor Tiberius. The process was as follows: King Charles I was given up by the Scotch for $200,000, Christ was sold for 30 denarii, what then was a denarius worth? In order to pursue the thread of such an association, one needs, anyway, only a definite quantity of historical knowledge, but this quantity must be possessed. But such knowledge is a knowledge of universal things that anybody may have, while the personal relations and purely subjective experiences which are at the command of an individual are quite unknown to any other person, and it is often exceedingly difficult to discover them.[212] The case is simplest when one tries to aid the memory of a witness in order to make him place single dates, e.g., when the attempt is made to determine some time and the witness is reminded of certain events that occurred during the time in question in order to assist him in fixing the calendar time. Or again, when the witness is brought to the place of the crime and the individual conditions are associated with the local situation. But when not merely single dates are to be associated, when complete events are to be associated, a profound knowledge of the situation must precede, otherwise no association is successful, or merely topsy-turvy results are attained. The difficulties which here ensue depend actually upon the really enormous quantity of knowledge every human being must possess in making use of his senses. Anything that a man has learned at school, in the newspapers, etc., we know approximately, but we have no knowledge of what a man has thought out for himself and what he has felt in his localized conditions, e.g., his home, his town, his travels, his relations and their experiences, etc.--However important this may be, we have no means of getting hold of it. Those associations which have physical expression are of importance only in particular cases. For example, the feeling of ants all over the body when you think that you have been near an anthill, or the feeling of physical pain on hearing the description of wounds. It is exceedingly funny to see how, during the lectures of dermatologists, the whole audience scratches that part of the body which is troubling the patient who is being described.