In this picture we see the great Italian philosopher Giordano Bruno standing in front of the Roman Inquisition. In the year 1600 he was burned at the stake at the Campo de' Fiori in Rome, murdered by the authorities because of his ideas. In a broad and comprehensive sense, the study of "Rejected Knowledge" covers all opinions that find themselves at odds with the world-views, values or basic assumptions that happen to be dominant and are considered normative in any given culture or society at any time of history. In a more restricted sense that is central to my own work, it stands for a large and complicated collection of traditions, ideas and practices since antiquity that have been dismissed as irrational or superstitious in Western society roughly since the period of the Enlightenment, as well as the continuation of those traditions up to the present day. How effectively these fields of Western culture have been written out of history and dismissed as unworthy of attention is demonstrated by the simple but telling fact that, without a single exception, all current labels for this domain as a whole (think of "esotericism," "mysticism," "magic," "the occult" and so on) carry connotations that somehow make them look questionable in advance - at least to many intellectuals and academics. Of course, to study the history of rejected knowledge does not mean we necessarily need to agree with esoteric world-views or defend their claims of knowledge; but it does mean taking them seriously and considering them in depth. It is impossible to do so for any length of time without asking oneself questions about the hegemonic discourses of exclusion that are typical of Western intellectual culture, and the subtle ways in which those discourses and their representatives have sought to present their own perspectives as "obviously serious" while dismissing their competitors as "obviously wrong." At Bruno's time you could be tortured and killed for having the wrong ideas, and sadly this remains the case in many societies up to the present. The social and intellectual elites in Western culture since the rise of modernity have been using different methods - less physically violent but at least as effective, if not more so - to marginalise and silence those who questioned its normative certainties. If you can manage to destroy your opponent's credibility by undermining his intellectual reputation and making him look weird or dangerous or crazy, you have effectively silenced him and taken away his power to challenge your own beliefs.
I have been studying the history of rejected knowledge for over thirty years now, driven by a deep conviction that precisely to investigate the unknown - to boldly go where no one has gone before - is the very essence of what the academy should be all about. The word wetenschap in Dutch (like Wissenschaft in German) is untranslatable in English, which forces us to choose between either natural "science" or "scholarly research" in the humanities, but it means literally the art or craft of searching for knowledge. Not wanting to know - not wanting to learn - about large parts of our cultural heritage, because we prefer to be confirmed in what we already believe, means abandoning that quest. Nevertheless, for a very long time the academy preferred not to know about the ideas and traditions that it rejected - paradoxical as this might be, for if you choose to remain ignorant about something, then how can you know so sure that it isn't worthy of your attention? The ideal of an open-ended search for better and more reliable knowledge - curiosity about the unknown - has always been my first motivation as a scholar of Rejected Knowledge. A second one concerns my deep belief that all human beings should have equal rights to freedom of thought and freedom of expression, that everybody should be be allowed to think or speak or write freely without fear of censure or worse. As a matter of principle, this puts me on the side of the heretics, freethinkers, nonconformists or misfits, and I will always defend their right to be heard. My third and final motivation has to do with the core humanistic values of open communication, of listening to the voices of others - even (or rather, especially) if they differ from our own - and of letting arguments and evidence prevail over mere emotion and personal prejudice. In short, all my work is inspired ultimately by three ideals: the open-ended search for knowledge and understanding, freedom of thought and expression, and the search for common ground and building bridges by means of listening to and learning from others than ourselves.
My main statement about Western esotericism as Rejected Knowledge is my book Esotericism and the Academy (2012). But of course, apart from my wish to create a space in the academy for serious in-depth discussion of what used to be excluded and dismissed, I am also motivated by the sheer joy of discovery. I love to learn about all those figures, ideas, practices, and traditions that few people have heard about but so often prove absolutely fascinating once we get to know them a bit better.
You can find many examples by clicking on articles and book chapters here.