Within your own self. The first layer of "within" can be easily understood. In between 2 successive thoughts is a state of "no thoughts". Can we like a scientist hypothesize that we are more than the thoughts? To prove or disprove it we need to observe/experience this state of thoughtlessness, initially they are just few milliseconds long but with practice will be seconds and minutes in length.

And remember if we are always observing through the filter of our thoughts then we are changing the thing being observed - just like in quantum physics "act of observing affected what is observed".