The idea that is all in your head is an idea that is mistaken, incorrect, without import. You might have the idea that you have heart disease. You are confident that you have it since both your parents did and you have been expecting to get heart disease ever since you turned 50. The doctors check you out and find no evidence of heart disease and plenty of evidence that your heart is healthy and behaving in a model, exemplary way. This notion that you have heart disease is "all in your head".
If you think you saw me coming out of the bank, wearing a mask and carrying a pistol in one hand and a cash bag in the other but I have a solid alibi (I was conducting a service in the local church at the time), the idea of me as a bank robber is all in your head. Several books that discuss our minds, our brains, our mental activity and its influence or lack of it briefly mention the use of the phrase "it is all in your head". They usually explain that our thoughts have consequences, whether or not they are grounded in reality. They ask the reader not to suspect the author of taking one's ideas or opinions or mental images for granted and not to believe that the author is saying "it is all in your head."
I have spent quite a stretch of time dealing with minds, my own and others. I am aware that my computer or furniture are not in your head. But, and it is a big but (!), the idea of my computer or my furniture is in my head and can be in yours. Might well be for a short time, since I just typed about it and you read my typing. Just as an elephant seal or a coconut might come to mind after reading those words. Our human heads are treasures or curses, maybe both. Just as the elk's antlers can be life-saving weapons or leave him to die entangled in tree branches, our heads can plan out an inspirational novel, guide us in a life-saving medical procedure or convince us wrongly that our neighbor hates us.
Writing down what we are thinking can help us sort out the good stuff from the fluff and the trash. Meditating for 5 or 10 minutes a day can help us get in touch with the thoughts we are afraid to think, or are embarrassed to think, or keep playing in the background. Lots of the world is not in my head, including much of what is in yours. But over time, I can develop a little clearer understanding of what is and some of the marvelous things I can do with my thoughts
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