Something that comes as a surprise to many when first approaching the Golden Dawn system is the prohibition against mediums, which was one of only a handful of things overtly frowned upon by the founders of the Order.
Of course, in modern groups this point might not be apparent, and in some cases the rule might not apply at all, but the original Order was very clear that mediumship was not permitted. In one of the early preliminary pledge forms that Candidates were required to sign, there was the following:
"The Chiefs of the Order do not care to accept as Candidates any persons accustomed to submit themselves as Mediums to the Experiments of Hypnotism, Mesmerism, or Spiritualism; or who habitually allow themselves to fall into a completely passive condition of Will; also they disapprove of the methods made use of as a rule in such Experiments."
This is explicit, criticising not only mediums themselves, but the manner in which they work. This is important, as the Golden Dawn tended to see such activities as a perversion of the Occult Science, a dangerous watering down of truer occult principles.
The Order did, of course, make use of esoteric practices that many would see as being similar to that of Spiritualism, such as the use of the Ring and Disc or the Tripod in the Theoricus Adeptus Minor sub-grade. The former bears some resemblance to the infamous Ouija Board, using a specially created Ring to douse above a complex Disc of numbers, symbols and letters in English, Coptic and Hebrew. The latter is clearly related to the popular performance of "table tipping," employing a special three-legged table and one or more operators.
If the parallel was not clear enough from the teachings of the grade, a paper on the Tripod describes the true occult performance as having been degraded by a growing phenomenon of the time, stating that it was "in recent years revived in a mistaken form among the Uninitiated under the title of Spiritualism and Table Turning."
The Victorian era of the Golden Dawn was also shared with the rise and peak of Spiritualism, which popularised the notion of séances and mediumship. To real magicians of great knowledge and experience this must have been harrowing to watch, in much the same was as the New Age movement or TV mediums are to modern magi.
But given that the Golden Dawn employed strikingly similar practises, on the surface, in some of its later teachings, what exactly is it about mediumship that is not tolerated? The key is passivity and the inherent danger and lack of knowledge and proper protocol employed in such activities.
Firstly, the medium opens up to whatever is out there, hoping to contact the dead, and yet not knowing if they will, and if what has been contacted is not their own subconscious, the wrong spirit, or something a little more sinister. It is epitomised in the phrase: "Is there anyone there?"
This is not the approach of the magician. The magician does not invite all and sundry into his or her temple in the hopes that something true and good shows up, but sets out with a goal in mind, selecting a spirit in question, employing its name, challenging it with various tests, and getting rid of it when desired or required.
These tests and the ability to banish are important, so much so that the Golden Dawn taught the Lesser Ritual of the Pentagram from the beginning, to Neophytes who would have to go through many more grades before beginning proper practical magical work.
The medium, however, usually lacks these safety measures. Not only do they call whatever is "out there" forth, they do not have any means to test what, if anything, shows up. There are no names or symbols or signs, no way to avoid self-delusion or the illusion of external beings.
The Golden Dawn advised safety regularly, not only teaching many ways to practise occultism in a balanced and safe manner, but coding in safeguards into the system itself. In the teachings on the Ring and Disc and the Tripod Mathers frequently warns of self-deception and obsession, offering various recommendations to limit this danger.
Yet the medium does not have these tools at their disposal, does not know how to deal with the force called upon, can merely open a door and not be certain of the ability to close it. Thus the first Power of the Sphinx, which must precede the others, is missing—and it is Knowledge.
And so a medium would be unable to truly join a Golden Dawn order, for they would immediately break their Oath, which states:
"I will not suffer myself to be hypnotised, or mesmerised, nor will I place myself in such a passive state that any uninitiated person, power, or being may cause me to lose control of my thoughts, words or actions."
The medium, who lets a spirit talk through them, would be unable to abide by this pledge, and such passivity is not what magic is about. While the Candidate of an initiation ceremony needs to be passive enough for the ritual to be successful, that is a far cry from the dangerous operation of handing over the reins to a spirit, especially a spirit that the medium knows next to nothing, or literally nothing, about.
The prohibition against mediumship was, therefore, not merely a product of the time period, but a genuine disapproval of unsafe occult practices made by those without the appropriate knowledge and experience to render the risks negligible. As such, it applies as much today as it did in the late 19th Century.