Why do we sometimes see things that are not there?
The phenomenon of “seeing things” comes in two varieties: illusions and hallucinations. If you see something but misinterpret what you are seeing, it is an illusion. If you see something that is unrelated to visual stimuli, then that is a hallucination.
Visual illusions are caused by misinterpreting visual stimuli. Since humans are visual animals, we make sense of our world primarily based on what we see. We are constantly taking in images and trying to fit them into the world we construct in our mind. Every time we see something new, we tend to compare it to things that we already know to make sense out of it. So, for example, visible heat waves rising in the distance might be interpreted as water in classic illusion known as a "mirage." Since faces are so important to our lives we tend to see faces even in inanimate objects, which is another classic type of illusion called "pareidolia." (Strictly speaking, pareidolia does not merely mean seeing faces, but seeing the familiar in the unfamiliar - but we usually use it for seeing faces.) There are many websites that give cool examples of pareidolia.
The reason behind seeing illusions is pretty well understood. People will try to make sense of the world in accordance to learned patterns, and if something does not fit into those patterns, we try to make it fit. For some cool examples and an explanation of how our mind tricks us, click here.
Hallucinations are less well understood. Hallucinations can happen at any time, but most frequently are caused by physiological conditions brought about by stress, exhaustion, drug intake, starvation, repeated rhythmic activity, etc. Hallucinations are typically individual phenomena, although there are many interesting cases of shared hallucinations related to "mass hysteria."
Visual hallucinations are a fertile field for investigation, and scientists still debate what exactly is going on during hallucinations. The famous neurologists Oliver Sacks dedicated much of his life to understanding hallucinations, and his work was still in progress when he died.
That being said, scientists are pretty sure that hallucinations are related to dreams. The physiological impetus that leads to hallucination seems to allow the brain to dream while you are still awake. In other words, you see the dream at the same time that you can interact with the ordinary world. Most people who are hallucinating can tell which are the hallucinatory images and which are not, although in cases of severe schizophrenia or under the influence of certain drugs (such as the daturas), the person is unable to distinguish. It is not unusual for people who recover from an episode of such hallucination to have little or no memory of it, much as people rarely have memories of their dreams.
In fact, it seems that forgetting dreams is necessary for maintaining a healthy sense of reality. If people are consistently awoken during their dreams and made to write down what they dreamed, they can start to show symptoms of schizophrenia. Schizophrenics often can remember their own dreams, and can't always tell whether their memories were of things that actually happened.
Finally, let me take a minute to implore you not to experiment with any of the daturas. They are extraordinarily poisonous, and people who have used them "recreationally" report the experience as extremely unpleasant.