We hear that statement when someone wants to dismiss a perception or an emotion as ephemeral, rooted in imagination rather than fact. On the other hand, quite literally everything IS in our mind ~ if we could not sense, imagine, deduce, quantify, or grasp it, it would have no reality for us (thought it might for others).
The above image is an example of something which briefly seems to fool our brains into seeing something which doesn't exist. Click on the image to enlarge, then stare at the red dot on the woman's nose for 30 seconds, then shift your eyes to a blank space. An optical illusion will appear, one which exists only in your mind.
Here's another sensory example of the central role of our minds ~ sex, and specifically orgasm. We tend to think of sexual pleasure as being physical, whether with a partner(s) or by oneself. But tactile stimulation alone isn't the key. Our brain is. The research of Kayt Sukel and others clearly shows that "we can orgasm without a single physical touch .... There are even some lucky individuals who are able to 'think off' ~ simply sit back, relax, and think themselves into an orgasm.' " One suspects that those devoted to tantric yoga may be well ahead of western science in exploring this realm. Sidenote: Sukel's book Dirty Minds: How Our Brains Influence Love, Sex and Relationships casts a broader net in revealing how it's all in our minds. What attracts us to one person, but not another? What causes that attraction to grow or to wane over time? The answers vary to some degree among individuals, but certain collective trends emerge.
So if our minds are critical to our understanding/experiencing the physical world, surely they are the very genesis of the metaphysical ~ philosophy, religion, the supernatural, the occult. We are wildly suggestible, easy prey to manipulation by magicians, politicians, televangelists, anyone who wants to sell a product or an idea. Perhaps, as we evolve, we are becoming less so. An article in Psychology Today suggests that atheism is in the process of replacing religion, for very specific reasons. "Atheists are heavily concentrated in economically developed countries, particularly the social democracies of Europe. In underdeveloped countries, there are virtually no atheists .... Atheists are more likely to be college-educated people who live in cities .... Atheism thus blossoms amid affluence where most people feel economically secure. But why?
"It seems that people turn to religion as a salve for the difficulties and uncertainties of their lives. In social democracies, there is less fear and uncertainty about the future because social welfare programs provide a safety net and better health care means that fewer people can expect to die young. People who are less vulnerable to the hostile forces of nature feel more in control of their lives and less in need of religion."
The history of the relationship between humans and mysticism suggests more than just economic reasons for our perceived need for religion. But the removal of fear and uncertainty, and the search for a rational understanding of the universe and our place in it, are surely central to the waning of religion.
Moving on. When regarding the subjects taught to us in school, it's difficult to identify one that was more abstract, more "in your mind" than math. The intricacies of algebra, geometry, trigonometry, calculus, or (shudder) statistics reward concentration (and at times a willingness to suspend disbelief) with "AHA" moments which blossom with ineffable beauty. The icing on the cake is that virtually every mathematical exercise from quadratic equations to logarithmic scales manifests in the physical world, especially in the natural world. Take Fibonacci numbers, for instance. The Fibonacci sequence starts with 0 and 1, and each number after that is the sum of the previous two ~ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, etc.
What? Something so apparently contrived as a Fibonacci sequence occurs in nature? Yes, over and over. To illustrate, here is a delightful triptych of short videos titled Doodling With Math: Spirals, Fibonacci, and Being a Plant ~ part 1, part 2, and part 3. Each segment is about six minutes long, and each proceeds in rapid-fire fashion, yet is easy to follow. The math and the artwork are fun, and you'll find light bulbs going off in your head like fireworks. It's all the project of Vi Hart, whose website is a playful treasure trove of sensual delights.
And it's all in your mind.