We introduce a ``prepotential''
F in quantum mechanics and show that the coordinate
x is proportional to the Legendre transform of
F with respect to the probability density. Inversion of the Schr\"odinger equation leads us to consider an
x-- $\ensuremath{\psi}$ duality related to a modular symmetry. The scaling of
x is determined by the ``beta function,'' suggesting that in quantum mechanics the space coordinate is a macroscopic variable of a statistical system with $\ensuremath{\Elzxh}$ playing the role of scale. The formalism is extended to higher dimensions and to the Klein-Gordon equation.
The story of wave-particle duality in quantum mechanics has been told a thousand times. This modest contribution is to get back to a few points, including memories of a discussion we had on a possible experimental evidence of quantum effects at macroscopic scale, but in a way totally unrelated to the experiments he and his group made quite later to exhibit a model of wave-particle duality. The more general question of interpreting/understanding quantum mechanics will be also scrutinized once more.