The brain is continually making predictions about the causes of the sensory information it receives, and it uses that information to update its predictions. In other words, we live in a “controlled hallucination” that remains tied to reality by a dance of prediction and correction, but which is never identical to that reality.
What you allow your milieu to feed into your mind is what you will be processing. It is what will carve the pathways of your brain. This is perhaps well-understood, in theory if not in practice.
But it is equally important to consider who and what you are sending output to.
It's a complex mess that'll ultimately require a ledger of interventions to remedy in full -- but last week, scientists presented an intriguing way to help alleviate the burden.