ABSTRACT
In order that this chapter may be clear to the reader, some understandings about terminology must be established. First, if we speak of ‘theories that were formulated in the past’, ordinarily we mean to leave open the question of whether they are still valid, and if we speak of ‘past theories’, ordinarily we imply that they have been discarded and that implies that they were defective and were supplanted by current theories. Nevertheless, in this chapter the latter term, used for the sake of brevity, will have the same meaning as the former expression. If a past theory is considered to be invalid, that will be stated explicitly.
