For the last 300 years science was intensely trying to discover impersonal laws of nature that govern fundamental reality. Classical and relativistic physics were (at least trying) to keep subjective elements at bay or left them to metaphysics. However, Quantum Mechanics (QM) might demand their inclusion to properly understand fundamental reality.

Quantum Bayesianism is one interpretation of QM (among many) that acknowledges the fact that our perception is always subjective. Thus, we can't exclude subjective elements from our theories (laws of nature) and that we've been including them all the time. It assumes that all our knowledge is epistemologic. In it's final instance it allows two conclusions;

  1. Reality (matter and laws governing it) exist but are a product of our consciousness. We co-create reality together with other conscious agents; a so called participatory realism,
  2. There is no reality; everything we perceive is just a product of our consciousness. (Yes, you understood correctly; there is no matter and hence no laws governing it. What we see with our own eyes and sense otherwise is painted by our consciousness but doesn't exist in reality.)

Q: Wait, these are such a preposterous suggestions! What made you think that could be the case?

If we manage to explain the world at the most fundamental (basic) level, we can in principle explain all the emergent events as well. That is, if we manage to figure out the exact laws that govern movement of fundamental particles (quarks, electrons, neutrinos etc.), we can in principle explain the movement of any matter that consists of them (cars, trees, planets etc.).

The best theory we have explaining the nature at the fundamental level is QM. Most of its theoretical predictions (where Classical and Relativity theory fail) have so far been confirmed. And not only that; our initial predictions matched our measurements (results) even when it would make more sense (upon later scrutiny) that they wouldn't (in a double-split experiment with single electrons they formed a wave pattern even though they were independent and so more likely to form a random pattern). Strange, right? It’s like we imagined the results and they were given to us.

Q: Ok, I'm sure you have an explanation for that?

In principle different interpretations of QM differentiate in how they explain the meaning of the wavefunction and it's apparent collapse (or lack thereof). Note that there is absolutely no consensus within the scientific community as to how we should interpret QM.

QBism as one possible interpretation suggests that any wavefunction represents our subjective degree of belief in the results of the observation.

Here is a brief explanation. In order to make predictions using QM, we have to, before each particular observation, come up with a mathematical model - so called wavefunction. The wavefunction is an equation that tells us the probabilities that we will get certain measurements. In order to come up with appropriate wavefunction, we usually insert the rules (natural laws) that we believe should apply and then "quantize" them.

While the laws that we insert might be impersonal, our choice among them is subjective. Subjective in a sense that our choice represents our subjective degree of belief as to what laws should in this particular experiment apply. Because our input is subjective it logically follows that a wavefunction is as well. It thus represents our subjective degree of belief in the measurements that we expect to obtain.

Q: Ok, so what's the big deal?

Insisting on such interpretation leads to conclusion that each of us is at least co-creating the reality by our consciousness each time we make an observation of reality (use our senses to obtain measurements). My assumption is that the reality we are experiencing is a convention (compromise) of all degrees of belief of different people (= agents = creatures with consciousness) about what the reality should be.

Q: Nah, this can't be true. If it was, then each of us would see a different picture of reality. Case closed.

The reason why we experience the same reality is because the rules that we have compromised on apparently include QM. The later knows a principle called quantum entanglement that demands the same result every time we observe entangled particles (our picture of reality is nothing else that billions of entangled particles), no matter how many times we repeat the observation. Therefore, all people (as entangled agents) inevitably experience the same reality when we're using our senses.

Q: That's crazy, I'm out.

Let's not stop here. We can go even further! It can be that all the matter and physical laws are a product of our consciousness. That there is no reality whatsoever. There might only be a world of ideas. The matter might exist because we agreed to it. And so does gravity that curves spacetime and "pulls" everything together. Craaaaazy, right!?