Like most clichés, this is easy to say, but hard to apply. It is imprecise and does not capture its own limitations. These three words don't move us forward; we shouldn't fixate on them; we must move beyond them.
Reality exists without perception. It benefits us to clarify the difference. Here are some clearer statements that reflect current philosophical and scientific knowledge:
1. We only perceive a small, incomplete, distorted portion of reality.
2. Human perception is a flawed but useful error-corrected simulation designed to help us survive.
2. Perceptions and beliefs strongly influence individual behavior.
3. Behavior is constrained by reality (perceived or not, believed or not).
4. Over a sufficiently long time scale, individuals and groups who understand reality have a survival advantage.
5. Perceptions can deviate from reality for arbitrarily long time periods.
> Perceptions can deviate from reality for arbitrarily long time periods.
According to the theory, this is not [contained within] reality.
What if the "obviously" correct model you were raised on is not correct? Possible for Newton's theories, but impossible for something even more complex?