Because opinio juris refers to the psychological state of the state actor—asking why the state behaved as it did—it can be difficult to identify and to prove. In practice, a variety of sources tend to be used to demonstrate the existence of opinio juris, including evidence such as diplomatic correspondence, press releases and other government statements of policy, opinions of legal advisers, official manuals on legal questions, legislation, national and international judicial decisions, legal briefs endorsed by the state, a pattern of treaties ratified by the state that all include the same obligation(s), resolutions and declarations by the United Nations, and other sources. In the Paquete Habana case (decided by the United States Supreme Court in 1900 on the question of whether small coastal fishing boats are immune from capture during wartime under customary international law), evidence of opinio juris included medieval English royal ordinances, agreements between European nations, orders issued to the U.S. Navy in earlier conflicts, and the opinions of legal treatise writers. Finally, the context, circumstances, and manner in which the state practice is carried out can also be used to infer the existence of opinio juris. As the ICJ stated in the North Sea Continental Shelf cases of 1969, "Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it." Nonetheless, a state's motives can change over time, and that it is not necessary that opinio juris be a significant impetus for each instance of action. As Judge Lachs wrote in a dissenting opinion in the North Sea Continental Shelf cases, "At successive stages in the development of the rule the motives which have prompted States to accept it have varied from case to case. It could not be otherwise. At all events, to postulate that all States, even those which initiate a given practice, believe themselves to be acting under a legal obligation is to resort to a fiction, and in fact to deny the possibility of developing such rules."