Why did Werner Heisenberg focus on observable quantities in quantum theory?

Werner Heisenberg’s decision to base quantum mechanics exclusively on observable quantities was a foundational shift in his 1925 work, "Quantum-Theoretical Reinterpretation of Kinematic and Mechanical Relations."1

Heisenberg adopted this approach while tackling the problem of spectrum intensities of the electron as an anharmonic oscillator. By focusing strictly on quantities that could be observed—such as the frequencies and intensities of light emitted by atoms—rather than relying on unobservable mechanical models of electron orbits, he sought to build a theory that avoided the limitations and inconsistencies of earlier atomic models.12

This methodological choice led him to develop a formalism based on noncommutative multiplication, which was subsequently expressed through matrix algebra by Max Born and Pascual Jordan. This framework became the basis for the new quantum mechanics, moving the field away from classical deterministic descriptions toward a more rigorous, empirically grounded foundation.13