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.
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.
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.