A thing can be simple and still fail, or complex and work beautifully. What matters is how its parts fit together and how the whole serves its purpose.
There are two dimensions to this fitness. Structural fit: the parts form a coherent whole. Functional fit: the whole suits what it is meant to do. When the two reinforce each other, we may experience that coherence as beauty.
Abstract reasoning helps us discover and create this fit. It allows us to look beyond a thing's existing form, identify the relationships that matter, and imagine how they might be arranged differently.
Sometimes the breakthrough isn't a better part, but a better relationship between parts.
Beauty may reveal fitness, but it cannot prove it. What appears coherent must still work in reality.