But we believe in the 'existence' of concepts such as energy, mater, infinity, time, space, jealousy, love, empathy, and humanity because:
- they are useful to us and
- we can make predictions using these concepts which can be verified in the real world out there.
But these concepts don't 'exist' in the real world, they only exist in the world inside our heads. This theoretical world can accommodate many concepts and stories - some of them can make predictions which can be verified, and some cannot.
Even some recent theories about the universe may not be verifiable in the real world - String Theory and Parallel Worlds for example. If it can be shown that a 'theory' is not verifiable then does it count as a theory? Maybe it's more of a story.
The world is made of stories.