Of these there are many kinds. First, elementals. They try to bar the astral plane against man. And naturally so, because they are concerned with the building up of the lower kingdoms, these elementals of form, the Rupa Devas; and to them man is a really hateful creature, because of his destructive properties. That is why they dislike him so much. He spoils their work wherever he goes, tramples down vegetable things, and kills animals, so that the whole of that great kingdom of nature hates the name of man. They band themselves together to stop the one who is just taking his first conscious steps on the astral plane, and try to frighten him, for they fear that he is bringing destructiveness into the new world. They cannot do anything, if you do not mind them. When that rush of elemental force comes against the man entering on the astral plane, he must remain quiet, indifferent, taking up the position: "I am a higher product of evolution than you are; you can do nothing to me. I am your friend, not your enemy, Peace!" If he be strong enough to take up that position, the great wave of elemental force will roll aside and let him through. The seemingly causeless fears which some feel at night are largely due to this hostility. You are, at night, more sensitive to the astral plane than during the day, and the dislike of the beings on the plane for man is felt more strongly. But when the elementals find you are not destructive, not an embodiment of ruin, they become as friendly to you as they were before hostile. That is the first form of the dweller on the threshold. Here again the importance of pure and rhythmic food comes in; because if you use meat and alcohol, you attract the lower elementals of the plane, those that take pleasure in the scent of blood and spirits, and they will inevitably prevent your seeing and understanding things clearly. They will surge round you, impress their thoughts upon you, force their impressions on your astral body, so that you may have a kind of shell of objectionable hangers-on to your aura, who will much obstruct you in your efforts to see and hear correctly. That is the chief reason why every one who is teaching Yoga on the right-hand path absolutely forbids indulgence in meat and alcohol.
The second form of the dweller on the threshold is the thought forms of our own past. Those forms, growing out of the evil of lives that lie behind us, thought forms of wickedness of all kinds, those face us when we first come into touch with the astral plane, really belonging to us, but appearing as outside forms, as objects; and they try to scare back their creator. You can only conquer them by sternly repudiating them: "You are no longer mine; you belong to my past, and not to my present. I will give you none of my life." Thus you will gradually exhaust and finally annihilate them. This is perhaps one of the most painful difficulties that one has to face in treading the astral plane in consciousness for the first time. Of course, where a person has in any way been mixed up with objectionable thought forms of the stronger kind, such as those brought about by practicing black magic, there this particular form of the dweller will be much stronger and more dangerous, and often desperate is the struggle between the neophyte and these dwellers from his past backed up by the masters of the black side.
Now we come to one of the most terrible forms of the dwellers on the threshold. Suppose a case in which a man during the past has steadily identified himself with the lower part of his nature and has gone against the higher, paralysing himself, using higher powers for lower purposes, degrading his mind to be the mere slave of his lower desires. A curious change takes place in him. The life which belongs to the Ego in him is taken up by the physical body, and assimilated with the lower lives of which the body is composed. Instead of serving the purposes of the Spirit, it is dragged away for tile purposes of the lower, and becomes part of the animal life belonging to the lower bodies, so that the Ego and his higher bodies are weakened, and the animal life of the lower is strengthened. Now under those conditions, the Ego will sometimes become so disgusted with his vehicles that when death relieves him of the physical body he will cast the others quite aside. And even sometimes during physical life he will leave the desecrated temple. Now after death, in these cases, the man generally reincarnates very quickly; for, having torn himself away from his astral and mental bodies, he has no bodies with which to live in the astral and mental worlds, and he must quickly form new ones and come again to rebirth here. Under these conditions the old astral and mental bodies are not disintegrated when the new mental and astral bodies are formed and born into the world, and the affinity between the old and new, both having had the same owner, the same tenant, asserts itself, and the highly vitalised old astral and mental bodies will attach themselves to the new astral and mental bodies, and become the most terrible form of the dweller on the threshold.
These are the various forms which the dweller may assume, and all are spoken of in books dealing with these particular subjects, though I do not know that you will find anywhere in a single book a definite classification like the above. In addition to these there are, of course, the direct attacks of the Dark Brothers, taking up various forms and aspects, and the most common form they will take is the form of some virtue which is a little bit in excess in the yogi. The yogi is not attacked through his vices, but through his virtues; for a virtue in excess becomes a vice. It is the extremes which are ever the vices; the golden mean is the virtue. And thus, virtues become tempters in the difficult regions of the astral and mental worlds, and are utilised by the Brothers of the Shadow in order to entrap the unwary.
I am not here speaking of the four ordinary ordeals of the astral plane: the ordeals by earth, water, fire and air. Those are mere trifles, hardly worth considering when speaking of these more serious difficulties. Of course, you have to learn that you are entirely master of astral matter, that earth cannot crush you, nor water drown you, etc. Those are, so to speak, very easy lessons. Those who belong to a Masonic body will recognise these ordeals as parts of the language they are familiar with in their Masonic ritual.
There is one other danger also. You may injure yourself by repercussion. If on the astral plane you are threatened with danger which belongs to the physical, but are unwise enough to think it can injure you, it will injure your physical body. You may get a wound, or a bruise, and so on, out of astral experiences. I once made a fool of myself in this way. I was in a ship going down and, as I was busy there, I saw that the mast of the ship was going to fall and, in a moment's forgetfulness, thought: "That mast will fall on me" that momentary thought had its result, for when I came back to the body in the morning, I had a large physical bruise where the mast fell. That is a frequent phenomenon until you have corrected the fault of the mind, which thinks instinctively the things which it is accustomed to think down here.
One protection you can make for yourself as you become more sensitive. Be rigorously truthful in thought, in word, in deed. Every thought, every desire, takes form in the higher world. If you are careless of truth here, you are creating a whole host of terrifying and deluding forms. Think truth, speak truth, live truth, and then you shall be free from the illusions of the astral world.