system
Given the following post, respond as if you're commenting on Reddit:
human
What is the difference between "seeing things" visually, mentally and hallucinogenically?

I can see things visually, and I can imagine things in my mind, and hallucination is visually seeing an imagined thing. I'm wondering how this works and a few questions in regards to it.

If a person who is currently hallucinating is visually seeing what his mind has imagined, then does that mean that while in this hallucinogenic state where his imagination is being transposed onto his visual image, then if he purposely imagines something else would it override his current hallucination with a new hallucination he thought up? It not, why? 

To a degree if I concentrate I can make something look to me as if it is slightly moving, or make myself feel as if the earth is swinging back and forth, subconscious unintentional hallucinations seem much more powerful however, why?
gpt
Actually the brain is not a passive receptor of information. 

When you get information from the eyes (an electromagnetic signal), it is compacted and sent through the optic nerve to the thalamus. 

There it meets a flow of information from the occipital cortex (where most of the visual areas are). Why is this? so the information from the eyes can be compared to the working model of the real world you are ALREADY predicting. You see with the occipital lobe to say it in a simple way. but it needs to be updated, the flow of information that the optic nerve provides help to update the model you have already in your brain. tweaking it to reflect the information being gathered. 

If we depended completely on the input from the eyes and we were a passive receptor of information the brain would not be structured like this. and we would need more brainpower to process what we are seeing. 

Most of what we see is just an useful representation of the world, but not that faithful. Remember the white with gold / black with blue dress? It has to do with how your brain decides to handle the available information. colors are not real also, it's something the brain makes up. 

Lots of things in our perception are actually illusions. and thats ok. the thing is when you hallucinate you are allowing yourself to process something as an actual perception that should have been inhibited. you have a filter that's not working correctly. Some scientists associate this to an overly active dopaminergic system that's teaching you that certain cognitive processes are reflecting the real world when they are not. it's like the filter has a low threshold to select what is real and what is not when thoughts emerge from what you are watching. the network is being overly active, generating representations that should not be there.

So to answer the question, the difference is the source. but illusions happen all the time, illusions are part of the visual processing system, but having a visual processing system that is too lax in the control of the network activation, leads you to see even more things that are not there.