We know from neuroscience that sense organs like the eye pick up light information and turn it into electrical signals. Parts of our brain, like the visual cortex, process these signals into an image of the world that we can make sense of. We have similar systems for hearing, touch, taste, and smell. Neuroscientists have also found systems in our brain that engage our attention called the Default Mode Network and the Direct Experience Network.
The default mode becomes active when our brain assumes that not much is going on and automatically switches our attention to the internal narrative that runs through unfinished business, imaginings, difficulties, or memories that happen to be near the surface. These unintentional thoughts are often about the past or future and can be driven by emotions. As soon as we enter one of these streams of thought, we disconnect from the here-and-now reality of direct experience. Our body and senses may be in the present, but our attention is somewhere else. For example, we could be walking on a beautiful coastal path, but rather than enjoying the sun and sea, our attention is drawn to a particular unresolved problem that distracts us from noticing and appreciating our surroundings.