There are three possible models built around what is fundamental (meaning it exists on its own and is not derived from something else) involving matter and consciousness.

• matter is fundamental, consciousness arises from complexity of matter (in a brain)

• matter and consciousness are both fundamental and exist side by side. Maybe something like a soul that is extra to the material body

• only consciousness is fundamental matter in the reality of an observer comes into existance by being experienced.

In the double slit experiment, a beam of light or tiny particles like electrons is fired at a wall that has two very narrow slits next to each other. When only one slit is open, the particles make a single bright band on a screen behind the wall, like throwing sand through one slot. When both slits are open, instead of just two bright bands, a whole pattern of many bright and dark stripes appears, which is what waves make when they overlap and interfere. The strange part is that even if the particles are sent one at a time, they still slowly build up the same stripe pattern, as if each particle behaves like a wave going through both slits at once. But if a detector is used to check which slit each particle goes through, the stripe pattern disappears and the particles act like little bullets, showing that observing them changes the outcome.