There is a branch of philosophy called Idealism. We think of that word to mean having noble thoughts and ambitions, but that is not quite what it means when a philosopher uses it, though it can include those things. In philosophy, it means the school of thought that considers reality to be in the mind rather than in matter or things. Idealism is thus the opposite of Materialism.
The Idealists are concerned with how information reaches us from the outside world. They maintain that the only way we can know anything at all is through the medium of the five senses - sight, hearing, taste, smell, touch. Anything that is in your mind apart from these sense-data, say the Idealists, must have its origin in your mind and not outside it. They say that a person who was born without any of the five senses would have no knowledge of the world at all.
One of the most famous Idealists of the past was George Berkeley, who was from Ireland. He said that nothing existed in our minds except what had already come into them in the form of sense experiences. The concepts we have in our thoughts, come from our experience. So we can only have the concept "cat" because we have seen a cat. Now, Berkeley went further, and said that the sense-data are all that you can ever know. So when you look at a table, you have the sense-data of the table but that's all. You can't know anything about what it really is, apart from what your senses tell you. This led Berkeley to say that to all intents and purposes, matter does not exist. The only existence it can have is in the mind of the observer.
So someone might ask, "What if I close my eyes, so I can't see the table? Does the table cease to exist?" A complete Idealist would say, yes it ceases to exist. But obviously this is a problem! The way Berkeley solved it was by believing in God, so when you shut your eyes, the table continues to exist, because God is observing it. In this way, the universe as described by Berkeley consists of God and the observer, who receives the sense impressions given by God.
This philosophy is what gives rise to speculations like: "If all that I can know about the world is through sense-data, how do I know that the information coming in is correct?"
Or: "How do we know we're not living in a virtual reality, like in the film, The Matrix?"
Or: "If a tree falls to the ground in a deserted forest, and no-one is there to hear it fall, does it make a sound?"
In space, there is no sound, because there is no air to carry the sound waves. But even when there is air, the movements caused by the falling tree only become sound when they reach an ear. So if there's no-one there, how can the tree make a sound? We think of the world as something outside and independent of us, but in fact, so much of "reality" depends on the observer.
This idea comes into the "thought experiment" called Schrodinger's Cat. Imagine an air-tight box with a lid, and inside the box is a cat. The box also contains a device which can release a deadly poison. (Sorry if you like cats, but this is only a THOUGHT experiment). The device has a random generator so it has an even chance of going off as not going off. Is the cat dead or alive? You can't know. The cat's existence is in a "probability-state" at the moment. The only way to find out is to open the box. Then the probability state ends and the status of the cat is determined, dead or alive. So it is the observer who has made this happen, before that the cat was in a state of probability. It's only by the process of observation that the cat is now dead (or alive).
A similar thought experiment is The Doomsday Device. This was in the writings of an astronomer, but I can't remember his name, so if anyone knows, please leave a comment? Imagine a Doomsday Device hovering above Planet Earth. If it goes off, it destroys everything on the planet, including humans and all life, with nothing surviving. If it doesn't go off, we survive. The Device has a random generator, like in Schrodinger's Cat. So we have an even chance of survival or total obliteration. What happens?
The astronomer says that we must survive. Even though it is an even chance, the Doomsday Device can never go off. Because, if it did, there would be no consciousness to observe it happening, and so it could not happen. Like the tree that makes no sound, or the cat which is only dead or alive because it is observed to be so, the Doomsday Device could not go off, because we would never know it had happened.