In his graphic novel From Hell, author, magician, and professional mad bastard Alan Moore wrote, "If there is one place that gods inarguably exist . . . it is in the human mind, where they are real, in all their grandeur and monstrosity." He has since spent a good deal of his time exploring that idea and its implications for spirituality and magick.
Beginning from the idea that thoughts are real events, and that concepts are real things (if not objecitive, physical things), Moore goes on to suppose that there is a greater Imagination which connects and enfolds the imaginations of individual humans, much as an individual person may have a home, but there is a world outside that person's door. From there, he suggests that there may be creatures native to that world, which are just as real as the thoughts and concepts of which they are composed. Magick, then, becomes the exploration of the greater Imagination, interacting with the beings that live there, and bringing back ideas and inspirations to be made manifest in life.
It is for articulating thoughts like these that Moore is one of my favorite living magicians. If you want to read more of his ponderings on this idea, I recommend his comic series, Promethea, now collected in five volumes.
In 2010, he expanded on this during a lecture in the Ecology, Cosmos & Consciousness series. Fortunately for us, the folks who organized that series recorded the lecture, and put it up on Vimeo. I've embedded the video below, but you can also watch it and others from the series at the EC&C page, here.
At about 1:04, Moore talks about a friend who worked to manifest his goddess as a visible presence in his daily life. The way he did this was to retreat from mundane society and spend as much time as he could visualizing this goddess as a presence in the world around him. Eventually, he found that he could see her in his imagination as clearly as he could see anything else with his eyes. They spoke and interacted as if she were a physical person in the room with him.
Around 1:08:40, Moore tells the story of the evening he spent with his friend and his friend's goddess. I have to say that it sounds to me just like any of the times I've spent with my friends who are mystics, whose gods were real (if subjective) presences in their lives. Well, minus the mushroom trip.
It occurs to me that this process isn't all that different from the way anyone who is of a religious or mystical bent brings their deity into their life. You imagine them with you, you talk to them, you listen for their response. You surround yourself with things that remind you of them, things you think they'd like to have around. Eventually, you feel their presence. Maybe you start to experience odd coincidences that seem like the sorts of things you'd like to think your invisible friend would do for you. Some people even start to hear and see their gods. Really, the only difference between the everyday mystic and Moore's friend is the retreat from everyday reality. Sometimes, not even that.