This seems a natural point to consider a few odd topic together. Firstly, equivalent theories. Consider the two theories “in front of me is a laptop made of plastic made of atoms” and “in front of me is an infinite number of infinitely small holographic projectors that act like a laptop no matter what I do to them or how I change them”. These make exactly the same predictions. I have no way of determining then which theory I should accept. I dont believe this to be a problem, I think that I'm not here talking about two theories. To my mind this is one theory expressed in two sets of words. We could just as easily say “en face de moi est un ordinateur portable”.
To a natural interpretation this is quite strange. Surely it makes all the difference in the world if the laptop is real or an illusion, but I would ask, what difference. Science tells us about perceptions, if there is a “way things really are” and it is not a perception, we can never know about it. Not in the sense that it's a hard problem or that we'll never be 100% sure. I mean we have exactly no clue, we cant even have an informed guess, because we have no information to base any ideas on.
We can know nothing that is beyond our sense data (I'll get onto metal constructions like logic later). It might be helpful to point out that my idea of what is metaphysical is a lot wider than normal. Normally it is said that the existence of gods is metaphysical, but that of chairs is not. I dont accept that we can have knowledge of chairs. Take my laptop, I can see patches of white and black that are explained almost perfectly by the scientific theory “in front of me is my laptop”, but also by the theory “I am a brain in a vat being fed sense data identical to those I would experience according to the laptop theory.” I dont claim that I can prove, or indeed that I have even the ability to think it probable, that my laptop is real.
We all have metaphysical beliefs. For instance, I believe my laptop exists. This is not true, nor is it false. It is better thought of as a statement about my emotions than about things in a “world out there”. If someone says that my laptop does not exists I dont think we really have any means of arguing about the fact, because there is no question of being right or wrong in the matter.4
Gods are tricky ideas. Many gods are purely metaphysical ideas with no scientific component at all. A god who starts the universe or who sits outside of it to judge or just to observe is totally metaphysical, a universe with him would look exactly the same as one without him. So we must be agnostic about such gods, we cannot have proof of them, but far more than that, we cannot even have an educated guess, there are no arguments for or against. So when I say “I dont believe in such a god” I am not right or wrong any more than I would be if I said “colourless green ideas sleep furiously”. There is no right and wrong because there is no truth in the matter.
Some gods however stray beyond this. A god that acts in the world is, by that fact, a part of the world. So we can distinguish a god that creates the world but has no more influence on it from a god who creates the world and then sets about populating it over a period of 6 days. This kind of intervening god is far more simple from a philosophical perspective. We can simply get a scientist to do an experiment and tell us with high levels of certainty if such gods exist. And, to my knowledge, none of them do, all stories of interventions by gods have been shown to be without evidence. The important problem for philosophy is how to combine rejection of aspects of supposed divine action which are false from ones which are metaphysical. Almost all religions combine both aspects.
The internal world
When I defined science I said we make theories about perceptions. Thoughts and emotions are perceptions. So in this sense we can expand the domain of science to include “subjective” ideas. The key idea is theory of mind. Normally the term theory of mind is used in the context of autistic people or children and used to discuss the idea that they cannot understand the behaviour of others. But I think we can interpret it far more literally.
Considering the object near our centre of perception (I.e. ourself), we consider as a scientific theory the idea that we can control our actions. The first theory is that motions of our bodies are exactly the things we think of doing. Then we observe ourselves breathing or our heart beating or some other automatic action. We also observe an imagined action that feels much like our feeling before acting which doesn't actually result in anything. So we have to slightly elaborate our theory, and this elaboration is going on right now in every neuroscience department and I'm not going to try and pre-empt this.
Looking at other people we see they act in similar ways to us. We observe that there are perceptions that look like . We also observe that these objects do things that are similar to how we act in a similar situation. Then we can propose that they, like us, have minds, memories etc which can be predicted based on their past. This theory works quite well and is the basis of almost all our knowledge of others. We simply ask “if I were standing where they are, and knew what they knew, and had their history and characteristics what would I do?”, this is normally a good prediction.
Other people and science
One problem that a lot of people have with a sceptical5 approach to the world is that we cannot possibly examine everything in the world. So suppose I am deciding if I should accept Newton's law of Gravity or General Relativity it's just not reasonable to expect me to wait for a solar eclipse, probably travel thousands of miles, and do the experiment myself. So we must postulate the theory of the reliable scientist. This states that we can, under some situations, expect that others are telling the truth about their own experiences, and that we can deduce from this things about our own perceptions. We can test this theory and compare it to the opposing theory that we should ignore other people in discussing scientific theories by doing experiments and comparing our results with those of others. This gives us a list of people who can, in general, be relied upon. We can go further, it is perfectly scientific to believe someone we know nothing about if they are recommended as reliable by someone whose past recommendations have been reliable. In this we we can build up and produce such concepts as peer review and repeated reproducible experiments.
The question of the existence of free will is an ancient one. And one that I believe will never be resolved. The question is, given how the universe is now and everything that has happened in the past. Is it possible that some of our actions are freely determined so that we could in theory do more than one different thing? Or are future events totally pre-determined? This I believe is metaphysical. Because there is only one direction of time, we only experience one version of events. If events are pre-determined then we experience just this one set of events, if other things could in theory have happened then there is some means by which exactly one outcome is selected and that happens. Either way our perceptions are exactly the same. There is literally no difference between a deterministic universe and a universe in which free will exists but which just happens to do the same thing.
4) I'm going to end up being loose with language. I'll probably say something stupid like “my laptop exists” is true when I mean to say that it is an accurate theory. You'll have to forgive me, and agree in advance to interpret that kind of thing in the context of the entire theory.
5) I mean philosophical scepticism. One of the most uninteresting and damaging things a first year philosophy student can be taught. There are different realms of epistemology and if you dont respect this you're just going to end up standing around like a nutter not knowing anything. "I know that I know nothing" is not indicative of wisdom, it means that your concept of knowledge is useless.