A look at Physicalism

Physicalism is a philosophical position that holds that everything that exists is fundamentally physical or material in nature. 

According to physicalism, all phenomena, including mental processes, emotions, and consciousness, can ultimately be explained in terms of physical processes or properties, such as those described by the laws of physics, chemistry, and biology. This view contrasts with dualism, which posits that there are both physical and non-physical (or mental) substances, and idealism, which holds that reality is primarily mental or immaterial.

Physicalism has its roots in the empiricist tradition of philosophy, which emphasizes the role of observation and experience in acquiring knowledge. It is closely related to materialism, which has a similar focus on the physical world but may differ in some nuances.

There are different forms of physicalism, including reductive and non-reductive varieties. 

Reductive physicalism posits that mental states and properties can be completely reduced to, or explained by, physical states and properties. 

Non-reductive physicalism, on the other hand, argues that while mental states depend on physical states, they cannot be fully reduced to them.

Physicalism has been a dominant position in the philosophy of mind and metaphysics, particularly in the 20th and 21st centuries. It is often seen as compatible with the scientific worldview and has been supported by advances in neuroscience and other sciences that study the relationship between the mind and the physical world.

Problems with Physicalism:

1. The hard problem of consciousness, posed by philosopher David Chalmers, asks why and how subjective experiences (qualia) arise from physical processes in the brain. While physicalism may offer explanations for the correlations between brain states and subjective experiences, it has not yet provided a fully satisfactory account of how and why subjective experiences emerge from brain activity.

2. Decoherence: The transition from the quantum scale to the macroscopic scale, where objects exhibit classical, deterministic behaviour,  this transition is not defined by a specific scale or size but rather by the emergence of classical behaviour from the underlying quantum phenomena.

The process that links the quantum and classical worlds is called "decoherence." 

Decoherence occurs when a quantum system interacts with its environment, causing the superposition of quantum states to break down and the system to behave more classically. In essence, the quantum effects are "washed out" by interactions with the surrounding environment, and the system starts to exhibit classical behaviour.

The scale at which constituent particles behave classically is determined by the extent to which decoherence takes place. Due to the large number of particles in a physical object and their constant interactions with each other and their environment, the quantum effects become negligible, and the object behaves as a classical object with a well-defined position, size, and shape.

There isn't a specific scale at which the transition from quantum to classical behaviour occurs. Instead, the emergence of classical behaviour in macroscopic objects, is a result of decoherence brought about by the interactions between particles and their environment. This process effectively "hides" the underlying quantum nature of the particles and gives rise to the appearance of an objective, classical reality.