Brain state theories of motivation/value are those theories that declare that the only thing that motivates intentional action, or the only thing of value (respectively), is that the brain itself be in a particular state.
If one then adopts the reasonable proposition that brain states are physical states, this means the only end of human action is to organize the matter and forces in the brain in a particular way or to put it though a particular sequence of changes. Nothing else matters; or, if it matters, it only matters as a means or a tool for getting the brain into a particular state.
This description may sound odd. However, it applies to many moral theories - some of which are widely held. Theories that state that all that matters is pleasure and freedom from pain, or all that matters is happiness are brain state theories. Pleasure and happiness are brain states. They describe states of affairs in which the matter and forces of the brain have a particular organization or structure or go thorugh a particular type of change.
Today, the most popular brain state theory holds that the only thing that matters is happiness. An egoistic interpretation claims that all of an agent's actions aim to promote his or her own happiness. An ethical alternative to this this account holds that what matters most is the greatest happiness for the greatest number. In both cases, the one and only concern - or legitimate concern - for human action is that the matter of the brain be organized in a particular way.
The primary argument in favor of brain state theories has been that it appears to be true. John Stuart Mill defended the greatest happiness principle by arguing that the proof that happiness is the sole end of human action is found in the observation that all people aim in all things for happiness.
All brain-state theories fall victim to the same type of objection.
If it were true that all that mattered was putting a brain in a particular state, then all of our energy should be devoted exclusively to putting brains in that state and keeping them there.
More specifically, we would fulfill these obligations or intersts by first getting a brain into the proper state (one in which a person is experiencing pleasure or happiness, for example) and, once it is in that state, lock it in that state and keep it there. Chop off the head, put it in a vat, and hook it up to a computer controlling a set of magnates that would preserve the brain in that state or loop it through a particular set of changes so that the demand that the brain be in a particular state is met.
Yet, few people actually express any interest ins such a fate. While they value happiness and pleasure, they do not like the idea of their brain being put in a state of happiness or pleasure and kept there.
Robert Nozick provided an objection to hedonism that can be expanded to cover all brain-state theories of value. Hedonism holds that the only thing of value to a person is their own pleasure and freedom from pain. Nozick asked us to imagine an experience machine that will give us pleasant experiences. We step in, plug into the machine, and spend the rest of our lives laying there experiencing pleasure.
This option has very little value to many people. Whatever reasons for action motivate them, they are not seen as reasons to act in such a way that one finds oneself in an experience machine.
One possible response is that people simply are not in the habit of seeing such a machine as producing pleasure. Though it can be described to them in this way, in fact they are familiar with other things in life (e.g., actually enjoying the company of friends and family) producing pleasure or happiness and not with getting it from a machine. "Common sense" denies the premise that a machine can produce such an effect and motivates the agent to stick with the trusted sources of pleasure or happiness.
However, if we postulate that the simplest explanation is usually the best, then the simplest explanation for the fact that people claim not to want to have their brains frozen in a particular state or perpetually looped through a particular set of changes is because they actually do not want it. The claim that they are deluded or employing a strategy that, in this case, motivates them to make the wrong choice needs more than the mention that it is a possibility.
This type of thought experiment can be expanded to cover all brain-state theories of value. Regardless of the brain state that one holds to be the sole holder of value or sole motivational end, we can imagine a machine that can manipulate the structure of the brain to put it in that state and keep it there. If the brain-state theory of value has any merit, then this is the ultimate end - this is the highest value. This is the thing that we have the most and strongest reason to do (whether we realize it or not).
The fact that so many people reject this end suggests that it is not, in fact, the sole holder of value or sole motivational end that some claim it to be.
Besides, why would brain states be the only things on the list? Brain-state theorists need to provide us with an explanation as to how it can be the case that the organization of matter and forces within the skull can have value, while the organization of matter and forces outside of the skull cannot. Or, where the organization of matter and forces outside the skull can be motivational ends, why they do not or should not. There is no relevant difference between matter within the skull and matter outside the skull that can account for this distinction.
This problem is compounded by the fact that our evolutionary fitness depends less on putting our brain in a particular state than on putting the world in a particular state. It does more harm to one's ability to reproduce to be happy while being eaten by a lion than it does to be unhappy while grazing peacefully on the savannah. "Don't get eaten by lions" seems to be a more important rule to follow than "Don't be unhappy."
Desirism is built on a theory of intentional action that is immune to these objections. This theory holds that motivation to perform intentional actions come from desires, where a 'desire the P' is a motivating reason to act so as to realize states of affairs in which P is true.
The reason that the people do not like Nozick's experience machine is because. for many of our "desires that P", the experience machine fails to provide is wotj a state of affairs in which P is true. A parent concerned for the welfare of her children is not going to be content to enter the experience machine to be fed the experience of safe and healthy children while her actual children suffer extreme agony. A doctor who values providing health care to people in desperately poor parts of the world would not be satisfied with the mere experience of such a state. They seek to make the propositions that are the objects of their desires true - and that is something the experience machine cannot deliver.
This theory answers the question of how the organization of matter within the skull can have value while the organization of matter outside the skull cannot by saying, "There is no reason. An agent can be motivated to realize states outside the skull just as easily as she can be motivated to realize stares inside the skull."
Evolution suggests that we will be descendants of beings that were very much concerned with realizing states outside the skull since their very survival depended on it.