How intelligent / smart are Dementors?

Question

How smart are Dementors?

From what I can see, they can be trained as guards, but they never show the ability to solve complex problems. (e.g. - They are never given a complex task of tracking down an escaped convict, just guard places where the convict might go.)

We never see them communicate, even though we know they can communicate with wizards. Even if they can communicate, that might not mean much. I can give orders to a pet dog (e.g. - Go get the ball. Come inside. Follow Joe junior.), but a dog can't have complex philosophical conversations.

Are they as intelligent as giants? As giant spiders? As small human children?

This quote implies they are less intelligent than dogs, giant spiders, and human children. A dog or child will know the difference between a person they are following and somebody else who just happens to get in the way.

A word of caution: Dementors are vicious creatures. They will not distinguish between the one they hunt and the one who gets in their way."

~ Dumbledore, The Prisoner of Azkaban

Answer 1

Dementors are probably of human intelligence

People generally treat Dementors as if they were capable of formulating complex thoughts. For example, when the Dementors search the Hogwarts express, Lupin has this to say:

"And Professor Lupin stepped over you, and walked toward the dementor, and pulled out his wand," said Hermione, "and he said, 'None of us is hiding Sirius Black under our cloaks. Go.' But the dementor didn't move, so Lupin muttered something, and a silvery thing shot out of his wand at it, and it turned around and sort of glided away.... "

—Harry Potter and the Prisoner of Azkaban

Lupin seems to believe that not only can the Dementor comprehend what he is saying, but it understands the relatively complex idea that they are not concealing Sirius Black.

Dumbledore does imply that they have a very alien nature, but in doing so affirms their intelligence:

"It is not in the nature of a dementor to understand pleading or excuses. I therefore warn each and every one of you to give them no reason to harm you.

—Harry Potter and the Prisoner of Azkaban

Not"Dementors are incapable of understanding you" but "It is not in their nature to do so."

Sirius speaks of the Dementors as believing that he is going insane:

Dementors can't see, you know...." He swallowed. "They feel their way toward people by feeding off their emotions.... They could tell that my feelings were less -- less human, less complex when I was a dog... but they thought, of course, that I was losing my mind like everyone else in there, so it didn't trouble them.

—Harry Potter and the Prisoner of Azkaban

Sirius doesn't even seem to be speculating, so perhaps the Dementors actually talked about this among themselves somehow.

Snape seems to think the Dementors will be pleased to have Sirius back in their custody:

All I have to do is call the dementors once we get out of the Willow. They'll be very pleased to see you, Black... pleased enough to give you a little kiss, I daresay...."

And while Snape may have been exaggerating, a Dementor certainly seemed to recognize Barty Crouch Junior, and sucked out his soul:

But Professor McGonagall’s voice drowned Fudge’s. “The moment that — that thing entered the room,” she screamed, pointing at Fudge, trembling all over, “it swooped down on Crouch and — and —”

—Harry Potter and the Goblet of Fire

Note that not only was this not what Fudge had ordered it to do (though he tried to justify it after the fact), but the Dementor did not attack anyone else as it passed through the castle. It personally remembered Crouch and wanted to Kiss him—"the moment" it "saw" him.

The Ministry can order Dementors to do rather specific things—for example, to attack Harry Potter.

“Yes, you have,” said Fudge forcefully, “and I have no reason to believe that your views are anything other than bilge, Dumbledore. The dementors remain in place in Azkaban and are doing everything we ask them to.”

“Then,” said Dumbledore, quietly but clearly, “we must ask ourselves why somebody within the Ministry ordered a pair of dementors into that alleyway on the second of August.”

—Harry Potter and the Order of the Phoenix

Most telling of all, of course, is that Voldemort was able to sway their loyalty away from the Ministry by offering them a more bountiful supply of prey, also implying some sort of intelligence.

“It is with almost equal regret that we report the mass revolt of the dementors of Azkaban, who have shown themselves averse to continuing in the Ministry’s employ. We believe that the dementors are currently taking direction from Lord — Thingy."

—Harry Potter and the Order of the Phoenix

Dementors clearly have a fundamentally nonhuman mindset. As Hagrid said, they care nothing for guilt or innocence as long as they can suck the happiness from people. As Dumbledore said, they do not accept excuses or justifications. Their allegiance is to whomever can offer them the most prey. The quote given by the questioner indicates that Dementors are vicious: they care little for whom they harm. But they are fairly consistently treated as having an high level of intelligence.

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_{Dementors are not listed as beings in Fantastic Beasts and Where to Find Them. However, this does not indicate that they do not possess sufficient intellect or other qualities to qualify as "beings," since there are many other creatures that are manifestly beings and are not mentioned in Fantastic Beasts and Where to Find Them, e.g. Veela and giants. It would seem that one possible criterion is "requesting status as beings," as mentioned in the entry for Leprechauns, something that Dementors would likely be supremely disinterested in.}

Answer 2

Within the HP world, at least according to Fantastic Beasts, we actually have a pretty solid way of classifying creatures according to their intelligence:

Grogan Stump, the newly appointed Minister for Magic, decreed that a “being” was “any creature that has sufficient intelligence to understand the laws of the magical community and to bear part of the responsibility in shaping those laws. Troll representatives were questioned in the absence of goblins and judged not to understand anything that was being said to them; they were therefore classified as “beasts” despite their two-legged gait; merpeople were invited through translators to become “beings” for the first time; fairies, pixies, and gnomes, despite their humanoid appearance, were placed firmly in the “beast” category.

Notably, Dementors are not described as "beasts" (creatures of lesser intelligence), nor are they classified as "beings" (creatures of higher intelligence). As such, the most obvious suggestion is that their intelligence falls somewhere between the two.

Dementors seem to operate largely on instinct, but with the ability to communicate at a basic level and to understand moderately complex commands (go to x, kill y). Smarter than a dog but dumber than a kid.

Answer 3

They seem intelligent enough to unionize. But then not intelligent enough to lobby in a way that doesn't get them attached to a political candidate whose manifesto is "Kill everyone who disagrees with me."