From The Notion Club Papers - an unfinished novel by JRR Tolkien. In Sauron Defeated - The History of Middle Earth Volume IX. Edited by Christopher Tolkien. 1992. Pages 195-7.
"'[Ramer said:] ...it is largely a rest-time, sleep. As often as not the mind is inactive, not making things up (for instance). It then just inspects what is presented to it, from various sources - with very varying degrees of interest, I may say. It's not really frightfully interested in the digestion and sex items sent in by the body.'
"'What is presented to it, you say?' said Frankley. 'Do you mean that some of the presentments come from outside, are shown to it?'
"'Yes. For instance: in a halting kind of way I had managed to get on to other vehicles; and in dream I did it better and more often. So other minds do that occasionally to me. Their resting on me need not be noticed, I think, or hardly at all; I mean, it need not affect me or interfere with me at all; but when they are doing so, and are in contact, then my mind can use them. The two minds don't tell stories to one another, even if they're aware of the contact. They just are in contact and can learn.
"'After all, a wandering mind (if it's at all like mine) will be much more interested in having a look at what the other knows than in trying to explain to the stranger the things that are familiar to itself.'
"'Evidently if the Notion Club could all meet in sleep, they'ld find things pretty topsy-turvy,' said Lowdham.
"'What kind of minds visit you?' asked Jeremy. 'Ghosts?
"'Well, yes of course, ghosts,' said Ramer. 'Not departed human spirits, though; not in my case, as far as I can tell..'.
"'Beyond that what shall I say? Except that some of them seem to know about things a very long way indeed from here. It is not a common experience with me, at least my awareness of any contact is not.'
"'Aren't some of the visitors malicious?' said Jeremy. 'Don't evil minds attack you ever in sleep?'
"'I expect so,' said Ramer. 'They're always on the watch, asleep or awake.
"'But they work more by deceit than attack. I don't think they are specially active in sleep. Less so, probably. I fancy they find it easier to get at us awake, distracted and not so aware. The body's a wonderful lever for an indirect influence on the mind, and deep dreams can be very remote from its disturbance.
"'Anyway, I've very little experience of that kind - thank God!
"'But there does come sometimes a frightening... a sort of knocking at the door: it doesn't describe it, but that'll have to do. I think that is one of the ways in which that horrible sense of fear arises: a fear that doesn't seem to reside in the remembered dream-situation at all, or wildly exceeds it.
"'I'm not much better off than anyone else on this point, for when that fear comes, it usually produces a kind of dream- concussion, and a passage is erased round the true fear-point.
"'But there are some dreams that can't be fully translated into sight and sound. I can only describe them as resembling such a situation as this: working alone, late at night, withdrawn wholly into yourself; a noise, or even a nothing sensible, startles-you; you get prickles all over, become acutely self-conscious, uneasy, aware of isolation: how thin the walls are between you and the night.
"'That situation may have various explanations here. But out (or down) there sometimes the mind is suddenly aware that there is a night outside, and enemies walk in it: one is trying to get in.
"'But there are no walls,' said Ramer sombrely. 'The soul is dreadfully naked when it notices it, when that is pointed out to it by something alien. It has no armour on it, it has only its being. But there is a guardian.
"'He seems to command precipitate retreat. You could, if you were a fool, disobey, I suppose. You could push him away. You could have got into a state in which you were attracted by the fear. But I can't imagine it.
"' I'ld rather talk about something else.' "
At this point in The Notion Club Papers, Ramer seems to be Tolkien's mouthpiece. I assume that the experiences he describes were, more or less, those of Tolkien (specific examples of this are confirmed in several footnotes by Christopher Tolkien).
This was written, according to the Chronology published in JRR Tolkien: a companion and guide by Hammond and Scull, at around the lowest point in Tolkien's life - associated with him doing the work of two Oxford Professorships at the same time (covering his move from the Pembroke chair of Anglo Saxon to the Merton chair of English Language and Literature), and also having to teach subjects in which he had no interest.
At any rate, it seems that Tolkien had direct personal experience of dreams in which he felt himself under attack by malicious minds.
C.S Lewis drew upon similar experiences in his work - most obviously in the Screwtape letters; and the work of Charles Williams is permeated with the phenomenon. These matters were discussed in The Inklings meetings.
So, seventy years ago it was apparently the case that highly prestigious and able individuals (who had and continue to have a major cultural influence) were openly discussing the 'supernatural' workings of evil purpose in the universe.
Seventy years later, to do so is - for mainstream public discourse, at least in the UK - taken to be evidence of craziness or simple-mindedness (the sort of thing that only 'fundamentalists' might engage in).
Is this progress? What discoveries were made over recent decades that rendered this kind of discussion absurd? Are we (as individuals, as a culture), nowadays, smarter, more insightful, wiser, more-learned, more honest than the circle of Tolkien and Lewis?
Or are we, perhaps, inferior in almost every respect - individually and culturally? So it seems.
In which case they are likely to know better than we; and we should be prepared to learn from them - or at the very least to take seriously what they took seriously.