What is the connection between the world inside our heads, and the world outside of it?
One possible answer is: life is a perception of your own reality.
It's a powerful statement, but what does it mean? How do our external and internal worlds fit into it? To resolve this, we can separate the idea into the three concepts that constitute it.
Firstly, your reality is the objective circumstances of your existence, the real world outside your head. It's how a rock might understand the world, if it could: as it truly is. Your emotions and experience are irrelevant to reality, because they disrupt the "real", thus it ceases to be reality. Reality is universal, shared by all who are subject to it, and the only external differences between individuals is that we don't all experience the same parts of reality. The real differences begin with how you perceive your own piece of reality.
The second concept, your perception is the mental filter that controls how you experience and understand reality. When someone is wearing a pair of metaphorical rose-coloured glasses, that's an example of an individual's perception. They might see someone in a good light because of how they perceive them, when in reality they could be rather different - their reality and understanding of reality are not the same thing. The brilliant part about perception is that you are in control of it - with enough willpower and self-awareness, you can choose a different way of perceiving reality, or change it on the fly. The way you perceive things is based upon your experiences and judgements - what has worked for you in the past is kept, what fails is rejected. As we wander through life, a healthy perception constantly adjusts to an ever-changing reality.
Finally, the third concept is your life: the end result of your reality being filtered by your unique and dynamic perception, forming your internalised understanding of the world around you. Everyone experiences life in a completely different way, and everyone perceives reality differently - these two things are entirely dependent on each other. What you call your life is completely unique, it being the result of your method of perception working to understand the reality that we all share.
Now, it might be said that we become less inclined to adjust our perception as we grow older. This might contribute to a decline of interest in new experiences and ways of thinking, instead slowly allowing ourselves to be cornered by routine. On the other end of the spectrum, people who are open-minded can be said to perceptively reflexive, mentally prepared to adjust their perception as they go to make better use of the world around them.
Consider the stereotypically greedy, wealthy person. They’re rich in money, but lacking in happiness. Their failure to grasp the abundance of their reality might come from their perception: if they're convinced that what they have is never enough, then they will eternally perceive reality in a way that makes them unhappy. On the other hand, it is entirely possible that someone living in poverty could be enormously happy, if they are able to perceive their world in a positive way. This might be an overestimation of their reality, but that can be a beautiful thing.
To reiterate: life is a perception of your own reality. Being aware of this system of creating your own life experience is immensely powerful, because it means you have the power to change it. It's important to understanding the need to be mentally reflexive. It means you can effortlessly adapt as the world does its best to throw you off balance, leading you to best experience of life possible.
"Peace is the result of retraining your mind to process life as it is, rather than as you think it should be.”