Besides the general principles of international law, there are also some specific principles of international environment law, that are: principles of prevention, common but differentiated responsibilities principle and sustainable development principle. Are these principles the principles stipulated in article 38 of Statute of ICJ? Or these are just customs? However, we should differentiate more on the nature of rules or principles as soft or hard istead of principles or non-principles. 

Question:

What are the two requirements for customary international law? What`s the difference between customs and customary international law?