It is my contention that the concepts which hide behind the terms constituting the title of this essay, have many traits in common and to a large extent deal with the same problem.
This, however, is not sufficiently admitted. Further theoretical and philosophical clarification is overdue.
In the first place the theorist in each field finds it difficult to confess to what extent the prevailing terminology is misleading; the more so because in spite of defective terminology he is—in a general way—in agreement with his fellow-specialists as to the object of their study.
In the second place the theorist in each field, finding it hard enough to follow the torrent of literature in his chosen specialty, tends to neglect developments in fields which however closely interrelated are considered to be distinct.