and Weinland adds that since motor learning is often overlearning, retention is further aided. The interference of emotional factors (e.g., love, fear, anger, insecurity) can cause forgetting, as can another manifestation of complete concentration—absent-mindedness. Forgetting can sometimes be attributed to blocking of the item for which recall is desired. And finally, when a task is completed, it is frequently forgotten because the mind has decided there is no further need to remember anything about it. Proceeding from investigation of the nature of remembering and forgetting, various authorities have attempted to devise principles, rules, and systems to aid in improvement of memory. Somewhere around 500 B.C. Simonides worked out a system of assigning things to be remembered, a position in space; a method also employed by Quintilian and Cicero. In the seventeenth century Henry Hudson applied a similar system involving association by visual symbol. A complicated digit-letter system was used as far back as the fifteenth century, appeared in Germany in the seventeenth century, and in England in the nineteenth century; this approach involves considerable practice and is applicable only to rote learning. It is also useful for theatrical type stunts. Successive-comparism systems—broad associations in a kind of chain systems—have been invented; these often require remembering as much inventive associational material as can be found in the already logically associated material of a well planned text. Another system was based on paired associates, like pen and ink, and