M-theory is not a theory in the usual sense.

M-theory is the only model that has all the properties we think the final theory ought to have, and it is the theory upon which much of our later discussion is based. We do not yet have a definitive answer to this question, but we now have a candidate for the ultimate theory of everything, if indeed one exists, called M-theory. It is natural to ask: Will this sequence eventually reach an end point, an ultimate theory of the universe, that will include all forces and predict every observation we can make, or will we continue forever finding better theories, but never one that cannot be improved upon?

In the history of science we have discovered a sequence of better and better theories or models, from Plato to the classical theory of Newton, to modern quantum theories. If two such physical theories or models accurately predict the same events, one cannot be said to be more real than the other rather, we are free to use whichever model is most convenient. But there may be different ways in which one could model the same physical situation, with each employing different fundamental elements and concepts.

When such a model is successful at explaining events, we tend to attribute to it, and to the elements and concepts that constitute it, the quality of reality or absolute truth. It is based on the idea that our brains interpret the input from our sensory organs by making a model of the world.

Stephen hawking book the grand design