How does consciousness affect the physical world?
I decide to have a piece of toast. This is a conscious decision: an event that occurs in my inner experience, in the realm of the subjective.
Shortly after, I walk into the kitchen, cut a slice of bread, put it into the toaster. Sometime later, I eat the toast. These are all objective events, in the external world.
In what sense did the subjective event cause the objective events? How do we explain the causal relationship (if any) between events in inner experience and events in the external world? In making the best explanations that we can of what happens in the objective world, to what extent do we have to include subjective events in our explanations?
Using only today's knowledge to explain the objective events, we will of course have to appeal to the internal event, the conscious decision, to say why my body did what it did. But is this only because we don't know enough about the objective world?
Armed with all the instrumentation that is in principle possible, and with all the physical, chemical, biological, and psychological knowledge of objective reality that we could in principle acquire, could we do more? Could we, in principle, explain every aspect of the objective events, every motion of every atom, every opening or closing of every molecular gate in a neural dendrite, every slap of my bare feet on the cold kitchen floor, solely in terms of other objective facts and events? Or will any attempt to do that necessarily fail at some point, requiring us to include subjective facts and events in our explanation? Either possibility leads to oddness.
If we can explain every objective event in terms of other objective events, it seems that my subjective decisions do not in fact make any difference in how the world behaves! Some people find this unacceptable, others have no particular problem with it. It seems, at least at first glance, to run contrary to our intuition that we have free will, but on closer examination the two questions may not be all that closely related (see "What is free will, and do we have it?"). On the other hand, it does relegate consciousness to the status of an observer, or at best of an agent all of whose orders are carried out by a different process, in which the giving of the orders plays no part.
Of course, if there is an invariable correlation between my making a certain inner decision and my neurons being in a certain objective state, and my neurons being in that state is part of the explanation of my body taking certain actions, we will be able to show a correlation between my subjective decisions and my objective actions. But, as we found in "How are we conscious of the external world?", correlation is a pale substitute for explanation. If my decisions are correlated with, but do not cause, my body's actions, am I still basically just a passenger here.
On the other hand, if the best possible explanation of the objectively-observable motions of my body cannot be explained entirely in terms of objective facts and laws, something very strange is eventually going to happen as we get better and better at explaining objective events. Eventually, we will come to a place where objective physics no longer works. Some phy