Thus the difference between a category of tenses, and a number of semantically similar morphemes (words or affixes) referring to time, is that the absence in an utterance of any such morpheme means that it is indifferent as to time, whereas the absence (if that is possible) of any tense-morpheme from an utterance in which the tense-category is used indicates a particular kind of time-reference (expressed by zero-affix) contrasted to the time-reference of all the other tense-morphemes.
- Harris (1940), Pag. 222