Occultism is the study of occult or hidden wisdom (forbidden knowledge). To the occultist it is the study of "truth", a deeper truth that exists beneath the surface: "The truth is always hidden in plain sight". It can involve such subjects as magic (alternatively spelled and defined as magick), alchemy, extra-sensory perception, astrology, spiritualism, lithomancy, and numerology. There is often a strong religious element to these studies and beliefs, and many occultists profess adherence to religions such as Gnosticism, Hermeticism, Luciferianism, Satanism, Thelema, and Neopaganism. While Christianity, Judaism, Hinduism, Buddhism, and Islam are generally not considered occult, some of their modern interpretations can be, as the interpretation of Hinduism within Theosophy or the various occult interpretations of the Jewish Kabbalah. Orthodox members of such religions are likely to consider such interpretations false; for example, the Kabbalah Centre has been criticised by Jewish scholars.
The word occult is somewhat generic, in that almost everything that is not claimed by any of the major religions can be considered the occult. Even religious scientists have difficulties in defining occultism. A broad definition is offered by Nicholas Goodrick-Clarke:
OCCULTISM has its basis in a religious way of thinking, the roots of which stretch back into antiquity and which may be described as the Western esoteric tradition. Its principal ingredients have been identified as Gnosticism, the Hermetic treatises on alchemy and magic, Neo-Platonism, and the Kabbalah, all originating in the eastern Mediterranean area during the first few centuries AD.
From the 15th to 17th century, these kinds of ideas that are alternatively described as Western esotericism had a brief revival. Alchemy used to be common among highly important seventeenth-century scientists, such as Isaac Newton and Gottfried Leibniz. Isaac Newton was accused of introducing occult agencies into natural science when he postulated gravity as a force capable of acting over vast distances. This revival of alchemy and other occult studies was halted by the triumph of empirical sciences and the Age of Enlightenment. "By the eighteenth century these unorthodox religious and philosophical concerns were well defined as 'occult', inasmuch as they lay on the outermost fringe of accepted forms of knowledge and discourse," and were only preserved by a few antiquarians and mystics. However, from about 1770 onwards, a renewed desire for mystery, an interest in the Middle Ages and a romantic temper encouraged a revival of occultism in Europe, "a reaction to the rationalist Enlightenment."
Based on his research into the modern German occult revival (1890–1910), Goodrick-Clarke puts forward a thesis on the driving force behind occultism. Behind its many varied forms apparently lies a uniform function, "a strong desire to reconcile the findings of modern natural science with a religious view that could restore man to a position of centrality and dignity in the universe. Since that time many authors have emphasized a syncretic approach by drawing parallels between different disciplines.
Direct insight into or perception of the occult does not usually consist of access to physically measurable facts, but is arrived at through the mind or the spirit. The term can refer to mental, psychological or spiritual training. Many occultists have studied science (perceiving science as an adjunct to alchemy) to add validity to occult knowledge in a day and age where the mystical can easily be undermined as flights of fancy. An oft-cited means of gaining insight into the occult is the use of a focus; a physical object, a ritualistic action (for example, meditation or chanting), or a medium in which one becomes wholly immersed. These are just a few examples of the vast and numerous avenues that can be explored.