The Restatement says of the source general principles of law that they are: "General principles common to the major legal systems, even if not incorporated or reflected in customary law or international agreement, may be invoked as supplementary rules of international law where appropriate."
As is stated in Brownlie's Principles of Public International Law (8th ed., 2012 at 37) :
"The rubric 'general principles of international law' may alternately refer to rules of customary international, to general principles of law....or to certain logical propositions underlying judicial reasoning on the basis of existing international law. This shows that a rigid categorization of sources is inappropriate. Examples of this type of general principle of international law are the principles of consent, reciprocity, equality of states, finality of awards and settlements, the legal validity of agreements, good faith, domestic jurisdiciton, and the freedom of the seas. In many cases these principles may be traced to state practice. However, they are primarily abstractions and have been accepted for so long and so generally as no longer to be directly connected to state practice. Certain fundamental principles of international law enjoy heightened normativity as peremptory norms." [emphasis added]