In this section, I examine why someone would not adopt each of the two approaches summarized above, and see if the reasons also apply to QBism.
Let us begin with behaviorist psychology. One reason to reject it as a fundamental approach may have to do with personal preferences. An investigator may recognize the effectiveness of behaviorism in treating a number of disorders but, influenced by the perception of his own mental states and their subjective importance, feel that this approach does not provide satisfactory psychological knowledge. But in addition to this personal or subjective reason, there is also an empirical one. No psychologist will deny that there are mental states. Behaviorists do maintain that there is much to do in practical and theoretical psychology without referring to mental states. But clearly, introspection helps to gain at least some insights that the study of stimuli and responses alone cannot get. There are empirical differences between predictions made on the basis of stimuli and responses only, and on the basis of introspection.
How does this translate to QBism? I will come back later to the question of personal preferences, focussing at this stage on empirical differences. As pointed out earlier, there are no empirical differences between the probabilistic predictions made by a QBist and the predictions made by someone who claims that the state vector represents the true state of a quantum particle. The empirical objection to behaviorism therefore doesn’t seem to apply to QBism.
This conclusion, however, rests on a far-reaching hypothesis. It will hold if quantum mechanics (or a suitable generalization to relativistic fields, strings and the like) is the ultimate theory of nature. This may be true, but it should certainly be challenged, both on the experimental and theoretical sides. One theoretical challenge specifically consists in attributing real states to quantum particles, e.g. hidden variables as in Bohmian mechanics [15]. Although Bohm’s original approach yields predictions in agreement with standard quantum mechanics, straightforward modifications of it predict empirical differences that can be put to the test [16].
Let us now turn to idealist philosophy. There is no doubt that idealism and solipsism are logically consistent and (at least for solipsism) conceptually simple views of the world. Moreover, there is no experiment that can distinguish between idealism and a realistic view of matter. Yet very few people, at least among scientists, are true idealists, let alone solipsists. Why is that?
I can see several reasons why most people are not idealists. The first one is that our intuitive feeling for reality is too strong. We just find it incredible that tables and chairs are not solid matter. The second one has to do with the “order” that we perceive in the phenomena. Again, we find it unbelievable that this order should be due to something solely in the mind. A third reason doesn’t apply to idealism in general, but to solipsism only. Even if there is nothing outside mind, we do not believe that our experience of other minds functioning, as it were, much like our own, could only be an artefact of our own unique mind.
It is important to point out that these reasons adduced against idealism and solipsism have nothing to do with logical requirements or the results of experiments. In the end, they boil down to personal preferences. The only objection I know to idealism and solipsism is, ultimately, “I don’t like it.” Such judgement is based on methodological or meta-empirical preferences, rather than on logic and experiments. My only way to convince idealists or solipsists to change their views is to bring them to share my personal preferences.
How can an argument resting on personal preferences eventually move a QBist? As pointed out earlier, most QBists do not deny the existence of quantum particles (i.e. electrons, photons, etc.). They deny that quantum particles have states, or that these states should be the object of science. I will address this group in the first place.
Suppose one believes in the existence of quantum particles. Then one can ask, “How can quantum particles be for quantum mechanics to be true?” Although some may claim that this question has no empirical meaning, it is hard to see how one could maintain that it has no logical meaning. I will now argue that in addition to having meaning, the question is also relevant.
More specifically, I can see three broad types of answers to the above question. I claim that all three are interesting and relevant, even to QBists.
The first possible answer to the question could be that there is a simple, coherent and intuitively appealing way to describe quantum particles that precisely yields the quantum formalism. This is what happens in classical mechanics, where the identification of \(m\), \(\mathbf {r}\) and \(\mathbf {v}\) with the mass, position and velocity of particles constitutes a noncontroversial way to interpret the formalism. Unfortunately, close to a century of research in quantum foundations has not produced such a simple interpretation. This, however, is no proof that none can be adduced, and research in that direction may still be worthwhile.
The second possible answer could be that there is no way that quantum particles can behave for quantum mechanics to be true. If that were the case, I claim that anyone believing in the existence of quantum particles would have to conclude that quantum mechanics, in spite of its empirical success, cannot be a satisfactory theory of nature.
Fortunately, this answer is not the correct one (at least in nonrelativistic quantum mechanics), since there are counterexamples. The simplest one is probably Bohmian mechanics which, irrespective of personal preferences, provides a clear and consistent way to describe fundamental particles in full agreement with quantum mechanics. Other counterexamples are many-world theories [17], transactional approaches [18] or consistent histories [19].
That brings us to the third possible answer to the question raised above. As I have just outlined, there are many ways the world of quantum particles can be for quantum mechanics to be true. But none has (for many people at least) the cogency that the standard interpretation of classical mechanics has in terms of masses, positions and velocities of particles.
The fact that none of these answers is appealing to QBists leads them to do away with these approaches and stick to the experience of agents. But for anyone who believes in the existence of quantum particles, that situation is problematic. How can one be comfortable with entities whose only known ways to behave are unbelievable? Doesn’t this lead to look for other avenues and seek new solutions to the problem?Footnote 2 If any known way by which the quantum mechanics of particles can be true raises problems, then these problems ipso facto transfer to QBism.