You reach over and exert yourself slightly: your mind tells your arm to tell your fingers to grab it--now the glass is in your possession. And now you drink the water.
Now let's say you were telekinetic. You wouldn't even feel the sensation of reaching out: you would just pull the glass toward your mouth. It was as if the glass was in your hand the entire time and you just chose that moment to move it to your mouth. It would, in fact, be like you were always holding that glass of water, so simple is the act of controlling it. It would be like the glass was part of your body all along. You were just moving it from one part of your body, which stored it as potential fuel, to another, which stored it as fuel-in-use.
We think of our body as one thing and our environment as another and we think of using our arms to pluck something from the world and using telekinesis to do it as different kinds of things.
If you and your whole race and entire culture were telekinetic and always had been, you wouldn't. Whatever was near what we normally think of as "your body" would be, in effect "your body" because you can get it whenever you want instantly.
And this extends to mental domination as well: if you, a person, see a rat run across your kitchen floor behind you, that's a thing. If you were a mind flayer, it's not even a thing. The rat runs and then you stop the rat, and the rat comes to you and you do what you like with the rat. The rat is part of you--as is the table, and the chair, and the kitchen knives because all are as under your control as your fingers and toes. It is all you.
A grell is a floating tentacle brain monster. The Grell Philosopher is a thing in D&D. Check it. But the extant sources are frustratingly if unsurprisingly mute on the content of said philosophy.
Luckily I'm here. And I have a blog.
Mind flayers, algoids, intellect devourers, and grell are actually all one species--known only as Philosophers or The Philosopher Species.
Whether these "species" are actually different kinds of bodies these creatures artificially grow or parts of the life-cycle of one species is not yet known.
All things below considered, it's possible they're not sure either.
What separates them from Cthulhoids is they're smaller and actually interested in humans and their kin, after a fashion.
Though their philosophies have many schools, the basics are these:
1) As above: that which you can control with mere effort of mind is you. Why is your human arm part of you? Because you can instantly move it with an impulse. That is why, to the mind flayer, the rat is part of the mind flayer's body, as are most things the flayer is aware of when it is not surrounded by other flayers. Humans have a mind/body/environment split, these creatures do not. They have: owned by me/owned by another/owned by none. Illithid, for instance, simply means "owned". All the universe is either Illithid (explored/owned: nearly synonymous) or Unillithid (unowned). For these creatures their "body" is not the little thing on the tabletop with the word "mind flayer" engraved on the bottom, but everything in the vicinity that is subject to their powers.
They see life very much the way a novel or painting might: with interacting expressionistic distortions and eccentric motifs that carry real power. A mind flayer who (due to your saving throw) can't immediately find a fantasy or fear to control you with sees you the way you see an ant that's scuttled under a cabinet too fast for you to get it in a jar: the ant can't move in the left-right dimension very fast, can't move in the up-down dimension at all unless it lucks out and finds a chair leg, doesn't really grasp that you basically surround it just by being near it and has no instinct telling it which way is a good way to go rather than a bad way, but it still might manage, out of sheer luck, sometimes, to scramble to a place you can't control. The mind flayer can deal with you later, but it might lack the energy or time to move the cabinet.
3) Now this sounds like a cliche, but you have to take in all the nuances: Because of 1 and 2 above, these creatures see recalcitrant elves and humans and dwarves and whatnot the way we see bacteria. If a human is in your dungeon, they're in you. In order to not hear this as just some Matrix bullshit you have to think about all the differences between handling bacteria and handling a war.
-Foe can be negotiated with
-Foe might not have to be subjugated, just made to stop fighting
-Foe is autonomous and can, in many circumstances, be left autonomous
-Foe, if left to its own devices after war, might be useful, helpful, etc
-Solution focuses on strategic or tactical thinking (marshalling existing resources)
-You don't negotiate with bacteria
-There is, however "good bacteria" and "bad bacteria". Good bacteria isn't just neutral or harmless, it helps you and is part of your body.
-Bacteria levels and kinds must be controlled, always, even when there is no war. It's just hygenic.
-Bacteria grows in neglected places. Period. It has no locus or origin point.
-Solution focuses on technological thinking (creating new resources to fix it) more than strategic thinking (deploying existing ones properly)
In short rather than waging war on humans, these creatures think of themselves as trying to cure themselves of humans or, perhaps, solve humans.
When they think of them at all, of course.
4) Thus their form of conflict is biomedical. 50% of the time you come into conflict with them, they're just studying you. They want to learn your epidemiology and report back. When they really want to kill humans, they tend not to just throw themselves at you--they invent something they intend to kill you with, just as we invent vaccines.
This is then given to the "body" (lair) of a Philosopher that is currently host to humans, in the hopes it will spread the cure to other humans. The invention might be a book or an idea or a disease.
Some say the beholder is the product of early Philosophers' experiments with magic.
6) The nearest analogue to their society would be something like the relation of someone on the internet (this would be a dim, eldritch and non-eudclidean internet) who hardly ever left their basement to all the people s/he knows and has never met on the internet. They receive erratic status updates of various levels of importance from acquaintances ("Mold on my east wall. Sister still has dwarves.") and usually care more about the information and ideas these others have than their physical loci. They are their ideas and are identified by them--as we know one another by our avatars, not our faces.
7) The philosophers have an extremely limited understanding of the continuity of physical space. They haven't even worked out that all physical places on the prime material plane (or even a given planet) are connected to each other, or at least that they're more connected to each other than any other places in any meaningful sense (unlike Medieval humans, who had the sort of opposite problem--figuring if you just went up enough you'd hit god). They're kind of shaky on what "planet" means. It's just not a very important concept to them.
When they see humans looking at a map and then sailing a long distance in a boat, they don't even know what that's about. The map looks, to them, like an extremely primitive schematic in one dimension lacking any of the vital psychographical detail that would be necessary to describe a journey from one place to another. They're kind of like Her if the only time Her ever talked to a human they were in the middle of a post-traumatic breakdown and from the middle ages.
8) FINALLY A GAMEABLE: Philosophers' lairs are full of stuff. Not that they value it--they receive so much stimuli from psionic connection they don't really get the point of, like, fabric or colors. But many are intensely interested in the culture of nonpsionic things as part of their research into their epidemiology. They'll have a grimoire or a finely-worked sword but they'll also have maybe a potted ficus tree or a play about a happy orphan because they're like "Look at this stuff, what is this? Crazy right?" The pointless baroquery of nonpsi spoken and written languages amaze and baffle them.
10) So, naturally, their homes (to them: their selves) often look like hellish 24-7 sanitariums where the line between science experiment, living radio transmitter, art installation and torture victim is very thin. They'll recommend having a male elf of 120-150 years being slowly crushed in a pile of chalcopyrite crystals in your basement at all times the way a nutritionist might recommend eating yogurt with the right lactobacilli in it. It's about self-respect.
11) If you meet two Philosophers in one place, they're usually a mated pair. Because of the way they view physical space, from their point of view it means they're pretty much melted together. They consider crowds and large groups of their own kind sort of disgusting. Cities are unheard of. That's why they're always hanging out in lonely places beneath the earth or on the edge of human settlements and are always surrounded by nonpsi minions ("limbs").
12) Since the Philosopher species can read your mind and the Philosophers see you as a disease, the state of mind they study you in is the one you're in right before you kill them and this state is always "Suddenly unpredictable". It is the erraticness of the bacterial races that alarms them the most. As creatures of science they seek a pattern, and their prey, at its most deadly, seems to have none. The less predictable, the more deadly. It is almost as if teams of 4-7 of them just stumble upon the Philosophers and, through uncoordinated action and without intelligent planning, assemble constellations of strike and spell that annihilate their host. This is of course the only way to defeat a foe who can read your mind, but this hasn't occurred to any of them yet since the idea of not planning is totally alien. Why would you ever not plan? You always know what's coming. In fact the very existence of unpredictability is a controversial concept among Philosophers.