John von Neumann proposed that the measurement problem involves a chain of interactions, starting from a physical system and moving through the measuring device to the observer's brain, and finally culminating in the conscious observer’s mind. He introduced a dualistic framework, distinguishing between two processes:
Process 1 (Collapse): This refers to the collapse of the wave function, a non-deterministic event that occurs when a measurement is made, turning a superposition of possible states into a definite outcome.
Process 2 (Unitary Evolution): This is the deterministic evolution of the quantum system according to the Schrödinger equation, where the system remains in a superposition of states until a measurement (or collapse) occurs.
Von Neumann did not explicitly claim that consciousness causes collapse, but he showed that collapse cannot be located anywhere in the physical chain, leaving the observer’s mind as the only remaining candidate. On this view, consciousness plays an essential role in resolving the quantum uncertainty, but von Neumann didn’t propose a mechanism for how this might work.
In 2007, Stapp extended von Neumann’s framework, giving consciousness a more active and fundamental role in the quantum process. He introduced the idea that conscious intention can influence physical outcomes through repeated acts of observation, employing the Quantum Zeno Effect (QZE). This is the idea that frequent measurement can prevent a quantum system from evolving, suggesting that conscious attention or focus can "freeze" certain quantum states and influence their evolution. In Stapp's interpretation, conscious choices, made by the mind, select among possible quantum states and repeated acts of conscious attention can stabilise certain outcomes over others, actively shaping the physical world. This differs from von Neumann, who saw consciousness more passively – simply collapsing the wave function without necessarily influencing the physical outcome with repeated decisions. Stapp argued that mental intention plays an active and causal role in determining which possibilities become reality. Note that he rejects classical dualism; instead, he proposes a psychophysical framework in which mind and brain interact through the structure of quantum measurement. Stapp sees the brain as a quantum system in which Process 1 corresponds to mental effort or the focus of attention, which leads to the selection of possible outcomes, and Process 2 is the deterministic evolution of brain states according to quantum laws. These can be influenced by the choices made during Process 1. Conscious decisions can therefore have a direct influence on the physical state of the brain, extending von Neumann’s abstract idea of consciousness collapsing the wave function into a concrete model of mind-brain interaction. This provides a model for free will in a quantum context, which Stapp further explored in Quantum Theory and Free Will: How Mental Intentions Translate into Bodily Actions (2017).