An observation that having many options to choose from, rather than making people happy and ensuring they get what they want, can cause them stress and problematize decision-making.

If you seek pleasure or happiness for the sole purpose of achieving it for yourself, you will fail. Instead, you must pursue other goals that will bring you happiness or pleasure as a side-effect.

The more similar two options are, the more difficult it is to decide between them, and the less consequential the decision becomes. A rational decider might find herself spending the most time on the least important decisions.

Intentionally blank page paradox: Many documents contain pages on which the text "This page is intentionally left blank" is printed, thereby making the page not blank.

Pilots who are psychologically unfit can bail out of combat duty, but anyone who attempts to do so establishes his sanity.

What happens when Pinocchio says "My nose grows now"? Basically, his nose would have to grow to make Pinocchio’s statement not a lie, but then it can’t grow otherwise the statement would not be a lie.

A word that does not describe itself is heterological. Does "heterological" describe itself?

When two countries each have nuclear weapons, the probability of a direct war between them greatly decreases, but the probability of minor or indirect conflicts between them increases.

Imagine that you are holding a postcard in your hand with the words "The statement on the opposite side of this card is true" inscribed on one side. That will be Statement A. The opposing side of the card states, "The statement on the other side of this card is false," when you flip it over (Statement B). A paradox arises when attempting to assign any truth to either Statement A or B: if A is true, then B must also be true, yet for B to be true, A must be untrue. On the other hand, if A is untrue, then B must also be false, hence A must inevitably be true.

A paradox involves a heap of sand from which grains of sand are removed individually. The dilemma is to think about what happens when the process is done enough times that only one grain remains: is it still a heap? Assuming that removing a single grain does not turn a heap into a non-heap. If not, when did it go from being a heap to not being one?

Paradox consists of a pair of statements, each of which ‘says’ the other is false.

The idea that people can perform tasks more effectively by focusing less directly on doing them, suggesting that direct intentional effort is not really necessarily the most effective approach to achieve a goal.

There should be many alien civilizations in our galaxy if there is nothing particularly special about Earth. We haven't discovered any proof of extraterrestrial intelligent life, though.

A law student agrees to pay his teacher after (and only after) winning his first case. The teacher then sues the student (who has not yet won a case) for payment.

If we don’t know what we don’t know, how do we know what to look for? Even if we happen to encounter what we don’t know by chance, we wouldn’t know it and wouldn’t know to inquire.

First noted by Marvin Stern and George Gamow, physicists who had offices on different floors. Gamow, who had an office near the bottom of the building noticed that the first elevator to stop at his floor was most often going down, while Stern, who had an office near the top, noticed that the first elevator to stop at his floor was most often going up. This creates the false impression that elevator cars are more likely to be going in one direction than the other depending on which floor the observer is on.

If you were to measure the coastline of a country by using a ruler on a globe, you would come out with a vastly different number than if you were to pace around the edge. The closer you look, the more wiggles and squiggliness you come across and instead of converging on a more accurate length, the coastline just keeps getting longer. The smaller your ruler, the longer it gets.

Would a ship still be the same if all of its wooden components were replaced during restoration?

A younger version of the physicist who is developing a time machine comes visit. The younger version builds the time machine using the schematics that the older version gives him, and uses it to travel back in time as the older version of himself.

On the one hand, Galileo proposed, there are square numbers. On the other, there are those numbers that are not squares. Put these two groups together, and the total number of square numbers must be less than the total number of square and non-square numbers together. However, because every positive number has to have a corresponding square and every square number has to have a positive number as its square root, there cannot possibly be more of one than the other.

Raven Paradox begins with the apparently straightforward and entirely true statement that “all ravens are black.” and is followed by statement that “everything that is not black is not a raven”.

If you say today is Opposite Day, then because of the rules of the game, today would be the opposite of what you just said i.e. not opposite day or a normal day. Instead, if you said it was a normal day, then it would be a normal day.

There is an indefinitely long "horn" that has an infinite surface area but a finite volume.

A law of classical logic stating that inconsistent premises always make an argument valid; that is, inconsistent premises imply any conclusion at all.

A person can simultaneously advocate two conflicting policy options, A and B, provided that the person believes that democratic decisions should be followed.

There is something "interesting" about every number.

Most people's friends are more socially connected than they are. In most social networks, most users have a small number of friends, yet a chosen few people have a large number of friends, which leads to the friendship paradox. Those social butterflies in the second group disproportionately appear as friends of those with fewer friends, which raises the average number of friends-of-friends in a similar manner.

A paradox exists when someone says, "This statement is a lie" or "This statement is false," because if it were true, the statement would be stating the truth. However, if the statement is accurate, it would reject the claim that it is a lie. The fact that this statement conflicts with itself shows that it can be both true and false.

The name comes from the idea that if a person travels to a time before their grandfather had children, and kills him, it would make their own birth impossible.

What happens when an unstoppable force meets an immovable object?

If you have a list of lists that do not list themselves, then that list must list itself, because it doesn't contain itself. However, if it lists itself, it then contains itself, meaning it cannot list itself.

The Sleeping Beauty problem is a puzzle in decision theory in which whenever an ideally rational epistemic agent is awoken from sleep, she has no memory of whether she has been awoken before. Upon being told that she has been woken once or twice according to the toss of a coin, once if heads and twice if tails, she is asked her degree of belief for the coin having come up heads.

The set of all sets would have its own power set as a subset, therefore its cardinality would be at least as great as that of its power set. But Cantor's theorem proves that power sets are strictly greater than the sets they are constructed from. Consequently, the set of all sets would contain a subset greater than itself.

It is a metaphor about the challenges of human intimacy. A group of hedgehogs seek to move close to one another to share heat during cold weather. They must remain apart, however, as they cannot avoid hurting one another with their sharp spines. Though they all share the intention of a close reciprocal relationship, this may not occur, for reasons they cannot avoid.

Everything is somewhere: so places are in a place, which is in turn in a place, etc.; this generates an infinite regression.

To get somewhere, you must first travel halfway; after that, you must travel the remaining distance in halves, and so on endlessly. Thus movement is not possible.

It is an illustration of a paradox in philosophy in the conception of free will. It refers to a hypothetical situation wherein a donkey that is equally hungry and thirsty is placed precisely midway between a stack of hay and a pail of water. Since the paradox assumes the donkey will always go to whichever is closer, it dies of both hunger and thirst since it cannot make any rational decision between the hay and water.

Consider a situation where there are two kids in the family, one of them is a boy. What is the chance that the other child is a boy, then? Given that there can only be one other child and that there are fairly similar odds of having a boy or a girl, the logical response is that the probability is 1/2. However, the possibility that the other child is a boy must actually be 1/3, not 1/2.

Biologist Richard Peto noticed in the 1970s that mice had a much higher rate of cancer than humans do, which doesn’t make any sense. Humans have over 1000 times as many cells as mice, and cancer is simply a rogue cell that goes on multiplying out of control. One would expect humans to be more likely to get cancer than smaller creatures such as mice. This paradox occurs across all species, too.

There is a barber who lives on an island. The barber shaves all those men who live on the island who do not shave themselves, and only those men.
The barber cannot shave himself as he only shaves those who do not shave themselves. Thus, if he shaves himself he ceases to be the barber. Conversely, if the barber does not shave himself, then he fits into the group of people who would be shaved by the barber, and thus, as the barber, he must shave himself.

A ball that can be cut into a finite number of pieces can be reassembled into two balls of the same size.

Also known as surprise test paradox is a paradox about a person's expectations about the timing of a future event which they are told will occur at an unexpected time. The paradox is variously applied to a prisoner's hanging or a surprise school test.

Imagine that there’s a “teletransporter” machine on Earth. It puts you to sleep, records your molecular composition, breaks you down into your constituent atoms, and relays that information to somewhere on Mars at the speed of light. At the receiving end on Mars, a machine recreates your body atom by atom down to the last detail. When that body wakes up, it will have all your memories and all the parts that make you who you are. Now, is the person on Mars still the same person as the one who entered the teletransporter on Earth?

A moving coin completes one full revolution after only going half the way around the stationary coin.

Should one tolerate intolerance if intolerance would destroy the possibility of tolerance?

Motion is not possible since an object in motion is always equivalent to an object that is not in motion.

The omnipotent being cannot create a stone it cannot lift.

In a bar, there is always at least one customer for whom the statement "If he drinks, everyone drinks" is true.

The premise states that a crocodile, who has stolen a child, promises the parent that their child will be returned if and only if they correctly predict what the crocodile will do next.

This paradox states that if you put a cat in a box with a poison that might kill it, at the end of an hour the cat has a 50% chance of being alive, and a 50% chance of being dead. According to quantum mechanics, since we can't see in the box to know if the cat is alive or dead, the cat is both alive and dead.

According to Zeno's argument, Achilles can never overtake a tortoise in a footrace if he gives him a head start. In order to pass the tortoise, Achilles must first reach the initial position of the tortoise.

The owners of fossil fuel resources are forced to increase resource extraction, which in turn intensifies global warming, as a result of an environmental policy that continually becomes greener.

Increasing the food available to an ecosystem may lead to instability, and even to extinction.