232 CALKINS : TIME AS RELATED TO CAUSALITY AND SPACE. evidently result in hopeless confusion, and in wrong theories of the two. The summary which follows includes the chief distinctions which this paper has tried to justify. Its first section has been added for the sake of completeness, though it involves the allusion to certain metaphysical principles which have not been discussed. UNITY AND MULTIPLICITY. A. I. Ultimate Unity. II. Fundamental Multipli- city. (Variously stated in different systems.) (a) Idealistic. The Absolute Self. Individual Selves. 'Ideas ' of the Absolute Self. (b) Realistic. 1. Matter or Force, or 2. ' Unknown Reality '. B. I. The Phenomenal Unity. II. The Phenomenal Multi- plicity. (a) Of the many (events or (a) Events (and moments). things) with each other ; Necessary Con- nexion. (b) Of each of the many (b) Things (and qualities). (things) with itself : Identity. The results of the closer study of the phenomenal category of necessary and universal connexion may be grouped together after a similar fashion. Phenomenal Unity of Neces- Terms of the Connexion, sary Connexion. 1. Irreversible. (a) Causality (concrete). (b) Time (abstract). 2. Reversible, that is. Reciprocal Determination. (a) Concrete. (a) External Objects. (&) Abstract. (b) Mathematical quanti- ties. Such a classification may at least suggest the possibility of a simple and accurate classification of principles often confused and as often falsely distinguished. 1. Irrevocable. (a) Events. (b) Moments. 2. Revivable.