I'm going to be even more controversial and contend that neither historical context nor primary sources should ever be necessary to legitimately criticise an idea. Primary sources and historical context are certainly valuable for the study of history and culture—but any idea worth its salt should always be able to stand on its own. By way of example, one shouldn't need to have studied the Rigveda, or understand the historical context of ancient India, in order to have reasonable ground as a proponent or critic of modern yoga practice.