We develop a physics of cortical dynamics in which a microscopic quantum realm and a macroscopicclassical realm coexist and are coupled at the synapse. Following Beck and Eccles, thetrigger for synaptic exocytosis is treated as a genuine quantum event governed by the position—momentum uncertainty relation (Part I); its environmental decoherence is a measurement eventin the von Neumann sense [1], and the population of such events seeds the noise of a classicalstochastic neural field whose Martin—Siggia—Rose—Janssen—De Dominicis path integral organizescortical fluctuations through a single parameter, namely Brain's Planck Constant; so that partof the cortical noise descends from it. Measurement event and perceptual decision areunified as one first-passage construction (Part III). We estimate Brain's Planck Constant from cortical parameters,validate the first-passage law numerically, obtain a falsifiable kinetic-isotope and temperaturesignature in synaptic release, decision error rate, and reaction-time variability, and read thearchitecture as a quantum—classical hybrid computer. We then derive (Part IV) the corticalfluctuation spectrum from a fluctuation—dissipation relation and compare it to the 1/f powerof EEG/LFP; develop the stochastic thermodynamics of the neural field, giving the brain’s effectiveBoltzmann scale and the Landauer cost of the synaptic measurement; and map brainstates, criticality and neuronal avalanches, and predictive-coding precision onto modulation ofBrain's Planck Constant. The quantum realm is real but confined to the molecular trigger; the classical realm governscognition; all dualist interpretation is kept outside the physics.