"Remember that you are this universe and this universe is you."

-- Joy Harjo, contemporary Native American (Muscogee) writer

If you can study a part of a process, event, or thing...
  human behavior
  the mind
  ecology
  the birth of a baby
  social organization
  truth
  reality

by
  measuring it
  observing it
  testing it under different conditions
  making a mechanical or computer-assisted model of it
  making a mathematical model of it
  observing things around it
  seeing how it reacts to and acts upon its environment

...you can make some assumptions about the whole nature of it.

...or can you?

Most fields of inquiry and research are based on this theory. It is so well understood to be true that we treat it as fact.

For example, we know that peoples' thoughts are caused by the mechanics of brain function. We have performed tests directly stimulating the brain, we have studied people, studied the chemistry and biology of the brain, we have studied people with portions of their brain removed or damaged...We know how the brain works.