§ 1. The conception of things as interacting leads to the two problems of Change and Causality. The paradoxical character of change due to the fact that only what is permanent can change. § 2. Change is succession within an identity; this identity, like that of Substance, must be teleological, _i.e._ must be an identity of plan or end pervading the process of change. § 3. Thus all change falls under the logical category of Ground and Consequence, which becomes in its application to succession in time the Principle of Sufficient Reason. § 4. _Causality._ Cause—in the modern popular and scientific sense—means the ground of a change when taken to be completely contained in preceding changes. That every change has its complete ground in preceding changes is neither an axiom nor an empirically ascertained truth, but a postulate suggested by our practical needs. § 5. In the last resort the postulate cannot be true; the dependence between events cannot be one-sided. The real justification for our use of the postulate is its practical success. § 6. Origin of the conception of Cause anthropomorphic. § 7. Puzzles about Causation. (1) _Continuity_. Causation must be continuous, and yet in a continuous process there can be no distinction of cause from effect. Cause must be and yet cannot be _prior_ in time to effect. § 8. (2) _The indefinite regress_ in causation. § 9. (3) _Plurality of Causes_. Plurality of Causes is ultimately a logical contradiction, but in any form in which the causal postulate is of practical use it must recognise plurality. § 10. The “necessity” of the causal relation psychological and subjective. § 11. Immanent and Transeunt Causality: Consistent Pluralism must deny transeunt Causation; but cannot do so successfully. § 12. Both transeunt and immanent Causality are ultimately appearance 158 BOOK III COSMOLOGY—THE INTERPRETATION OF NATURE CHAPTER I INTRODUCTORY