There exist external observers which cannot be treated within quantum mechanics, namely human (and perhaps animal) minds, which perform measurements on the brain causing wave function collapse.
Treating the entire physical universe in this unified way provides a conceptually simple and logically coherent theoretical foundation...
This was partly because he was embarrassed that "consciousness causes collapse" can lead to a kind of solipsism, but also because he decided that he had been wrong to try to apply quantum physics at the scale of everyday life (specifically, he rejected his initial idea of treating macroscopic objects as isolated systems).
This interpretation relies upon an interactionist form of dualism that is inconsistent with the materialism that is commonly used to understand the brain, and accepted by most scientists.
The only form of interactionist dualism that has seemed even remotely tenable in the contemporary picture is one that exploits certain properties of quantum mechanics.
First, some [e.g., Eccles 1986] have appealed to the existence of quantum indeterminacy, and have suggested that a nonphysical consciousness might be responsible for filling the resultant causal gaps, determining which values some physical magnitudes might take within an apparently "probabilistic" distribution...
This is an audacious and interesting suggestion, but it has a number of problems... A second way in which quantum mechanics bears on the issue of causal closure lies with the fact that in some interpretations of the quantum formalism, consciousness itself plays a vital causal role, being required to bring about the so-called "collapse of the wave-function."
[5] For example, Roger Penrose remarked: "[T]he evolution of conscious life on this planet is due to appropriate mutations having taken place at various times.
[13] However, the argument was shown to be invalid because an interference pattern would only be visible after post-measurement detections were correlated through use of a coincidence counter;[14] if that was not true, the experiment would allow signaling into the past.
Researchers found that 6% of participants (2 of the 33) indicated that they believed the observer "plays a distinguished physical role (e.g., wave-function collapse by consciousness)".
However, this is because he understood the probability function as an artifact of human knowledge: he also argued that the reality of the material transition from "possible" to "actual" was mind-independent.
[24] Albert Einstein, who believed in realism, and did not accept the theoretical completeness of quantum mechanics, similarly appealed for the merely epistemic conception of the wave function: [I advocate] that one conceives of the psi-function [i.e., wavefunction] only as an incomplete description of a real state of affairs, where the incompleteness of the description is forced by the fact that observation of the state is only able to grasp part of the real factual situation.
[26] He believed quantum theory offers a complete description of nature, albeit one that is simply ill-suited for everyday experiences – which are better described by classical mechanics and probability.