An illusion is anything which is not quite as it appears to be at face value. Illusions are just mental concepts based on perceptions of what you assumed were real through in reality had no actual structure or substance but in terms of perception otherwise appear to have actual structure and substance. The prime example is the Full Moon illusion whereby the Full Moon on the horizon or near the horizon appears much larger than when it is seen much higher in the sky or overhead. Of course it really is just an illusion. Here are some further examples of what IMHO are probably illusionary in that these concepts are not quite what they appear to be.
Really Real Reality Is An Illusion: Really real reality is just an illusion if we are really virtual reality beings ‘existing’ in a simulated landscape. That is to say, we are just programmed software in a sort of video game that’s been constructed by an entity or entities unknown. IMHO the odds that this is our (virtual) reality is quite high.
Superposition of State and the Collapse of the Wave-Function Are Illusions: It’s an illusion to have something that both is, and is not, at the same time and in the same place (i.e. – a prime example being Schrodinger’s Cat).
Depth Is An Illusion: Depth is an illusion is a possible three-fold way. Firstly, if we live it what’s termed the “Holographic Universe” then we no more have depth than the images in a hologram. Secondly, all of your perceptions, your version of reality, is totally within your own mind and is actually just a 2-D (up – down; left – right) image that your brain / mind generates. Thirdly, just like the images you see on a TV screen or on the silver screen or on your computer monitor or in a painting / picture are all really 2-D (depth is just an illusion of perspective and of your perception), if we in turn are just simulated beings in a virtual (video game) reality then we are obviously 2-D beings with again depth being a manufactured illusion. We’re no more 3-D than the characters in our computer simulations.
Dimensions Are An Illusion: Just like latitude and longitude, the concepts of up – down (top – bottom); left – right; and forward – backward are all illusions on the grounds that they have no actual structure nor any substance. They are inventions pure and simple to assist us in navigating our way around town, around the Earth; and around the Universe. Because the idea of dimensions are illusionary, the entire concept of String Theory must also be illusionary in that String Theory is dependent on the concept of there being really real actual extra dimensions.
Space Is An Illusion: You cannot create space or manipulate space so space is just another one of those mental concepts we entertain that helps us come to terms with life, the Universe and everything. Space is that mental container or box we imagine that contains the real stuff of the Cosmos like matter and energy in all its myriad forms.
Time Is An Illusion: Time is just change, a measurement of rate of change. Change is nothing more than motion. No motion equals no change equals no time. It’s meaningless to talk about time in the complete absence or change / motion. Measurements and units of measurements are just human inventions; mental concepts. In the absence of human beings, the concept of an hour is meaningless. An hour has no structure or substance; no mass or density; no colour or texture or hardness or electric charge and no other chemical or physical property either.
Space-Time Is An Illusion: If both 3-D space and time are illusions, then clearly the concept of space-time is also an illusion. However, it does make for an elegant mathematical structure that gives an illusionary explanation to how much of the large-scale Universe functions.
The Accelerating Universe Is An Illusion: An expanding Universe that is accelerating in that expansion rate is an illusion because it violates one of the most basic laws in all of physics, that being the conservation of matter / energy. If the Universe is accelerating in its expansion rate then additional energy has to come from somewhere. You can’t create additional energy out of nothing. Yet, that’s exactly what cosmologists backing this observation do. They invent extra energy to drive this acceleration, so-called ‘dark energy’, a hocus-pocus concept invented out of less than thin air.
Mental Reality Is An Illusion: As eluded to earlier, all of your reality is generated by your brain / mind via the electrical signals it receives from your sensory apparatus. There’s no room in your brain / mind for a life-sized 3-D reality. External reality cannot fit inside your skull. Your brain / mind compresses your external reality down into a miniaturized 2-D mental version of your alleged external reality.
Sense of Self; Self-Identity Is An Illusion: Your sense of self-identity, consciousness and sense of free will is absolutely so strong that you have absolutely no doubt that they exist and have a reality internal to yourself – whatever that actually is if it is anything at all. However, the central mystery is, for something so obvious, why are these reality ‘facts’ so elusive and hard to pin down? Your body has substance and structure. Your brain has substance and structure. Your sense of self-identity, consciousness and sense of free will do NOT have any substance or structure to them, so do they actually exist at all? Are you, that is your ‘inner you’, just an illusion? ‘You’ the ‘you’ that you think exists inside yourself, inside your body (i.e. – inside your brain) doesn’t actually exist at all. ‘The self’ is illusionary. You are basically just a mish-mash of millions of interconnected neurons, genes and memes. No more, no less. There is no real nebulous ‘inner you’.
Sleep Is An Illusion: Sleep is an illusion on the grounds that you are not an organism (singular) but a colony of billions of organisms – your body cells. Now if none of your body cells sleep 24/7/52, how can it ever be said that you are asleep.
Free Will Is An Illusion: Going back to the concept of you being a colony of body cells, if each and every one of your body cells has no free will, it’s difficult to see how any conglomerate of them can have free will. Further, there’s no experimental evidence anyone has free will, quite the contrary in fact. Further, if we exist in a 100% deterministic cosmos; a clockwork universe, then all outcomes are predetermined and thus there’s no free will. If the cosmos is entirely random then again you have no free will as all your decisions are random. Theologically, if God (see below) is all-knowing then you cannot have free will.
God Is Not Only An Illusion, But A Delusion: Evolutionary biologist Richard Dawkins has covered this very nicely and in depth and there’s nothing I can add here that hasn’t already been said on the subject.