Source: [[Reference Notes/Scale]] > To varying degrees, *all* theories and models are incomplete. They need to be continually tested and challenged by increasingly accurate experiments and observational data over wider and wider domains and the theory modified or extended accordingly. This is an essential ingredient in the scientific method. Indeed, understanding the boundaries of their applicability, the limits to their predictive power, and the ongoing search for exceptions, violations, and failures has provoked even deeper questions and challenges, stimulating the continued progress of science and the unfolding of new ideas, techniques, and concepts. > A major challenge in constructing theories and models is to identify the important quantities that capture the essential dynamics at each organizational level of a system. Eg, when trying to understand the laws of planetary motion, the masses of the planets are important, but not their colour. However, if we zoom into the Earth, we find that the Earth's "blue" is made up of a complex ecology. At that scale, different things become relevant. Science proceeds by developing toy models and zeroth order approximations, which are then refined through experimentation and observation.