Why do we assume a physical world really exists? How do we not know that all there is are ideas and perceptions and we are deceived? Descartes using his hyperbolic doubt method had to doubt everything that could be doubted. He said that we have to appeal to our intuition to grasp what reality actually is, but what if it were the case that the evil demon is not just deceiving us but controls what thoughts we have in our mind? Do we know how thought arises in our consciousness? What's to say the evil demon doesn't send thoughts to my mind in a continuous stream?
This brand of radical doubt was in use in Buddhism since over two and a half thousand years to facilitate detachment and mental calming. The wisdom behind it was that humans are driven by instinct to strive, fight, crave etc. so by being aware of the manipulative nature of base instincts they can be curbed. This leads to the idea that appearances are generally deceptive, summed up in the concept of Maya.
It is all like a mirage in which springs of water are seen as if they were real. They are thus imagined by animals who, made thirsty by the heat of the season, run after them. Animals, not knowing that the springs are an hallucination of their own minds, do not realise that there are no such springs.
A Buddhist Bible, page 49
As for using "intuition to grasp what reality actually is", this is indeed all you have to go on. You can never be sure at which point you may be completely mistaken about something, but you can take a punt. Still it's a shot in the dark and you might be wrong.
Normally these would be considered separate questions, and you would be asked to ask them in separate posts. But there is one response to them that ties the two together
Do we have free will?
This question on itself is one of the major topics of metaphysics, and doesn't have a conclusive answer answer yet. Many have deemed the problem intractable, that it is something we humans can never know. Harvard philosopher Robert Nozick says in "Philosophical Explanations":
""The problem is so intractable, so resistant to illuminating solution, that we shall have to approach it from several different directions. No one of the approaches turns out to be fully satisfactory, nor indeed do all together." -- Philosophical investigations - p292
For consistency's sake I will mention the three main positions on freewill:
- Libertarian free will: The world is not determined, and we have freewill.
- The world is determined, and we do have freewill. The two facts are compatible, and this position is known as compatibilism. It does require that the definition of free will be changed.
- We don't have free will.
And is the world we observe an illusion?
How do we not know that all there is are ideas and perceptions and we are deceived?
but what if it were the case that the evil demon is not just deceiving us but controls what thoughts we have in our mind? Do we know how thought arises in our consciousness? What's to say the evil demon doesn't send thoughts to my mind in a continuous stream?
Daniel Dennett is a compatibilist: He believes that freewill and determinism are compatible, because we are free to make our own decisions, without violating causal determinism.
In his book "Consciousness Explained" (Chapter 1 - How are hallucinations possible? - Section 1 - The brain in a vat), he the offers an reason for why it is unlikely that the world we observe is an illusion. Although his example concerns evil scientists dealing with a brain in a vat, it can apply just was well to the case of a metaphysical evil demon controlling all of our thoughts and actions. His reasoning goes like this:
- It is easy to simulate reality for a passive person. It would be easy for the scientists, or evil demons, setting up the inputs to the person's brain or mind to create the illusion for someone of sitting on a beach doing nothing, sitting on a moving train or boat, watching the scenery move by.
- People in real life however, are not passive, and the subject would soon realize that something is amiss if they are not capable of doing anything other than observe the surroundings, or the number of actions they were able to take were very limited.
- For a person to be truly fooled by the simulation, they should be able to freely interact with the world around them. They should be able to get up and walk away from the beach, or get off the train at whichever station it chooses. This leads to an infinite number of possibilities, and it would be impossible for the scientists (or demons with magical but still limited resources) controlling the inputs to the person to account for all these possibilities. A metaphysical demon might be able to do so, but such a demon would have to be omniscient, a god in its own right, to be able to pull off such a feat.
- Therefore unless the demon was an omnipotent deity, the continuous stream of thought and sensations that they are sending will be limited in scope compared to the option in a real world. Sooner or later the person will realize that something is wrong with the world they inhabit, because they will inevitably be stuck with a limited amount of choices in their actions.
To summarize the argument: For a person able to decide among multiple courses of action, the number of future courses of action in the real world is so large as to be practically infinite. Unless the scientist or demon was omnipotent (in which case all bets are off anyway), the simulated world/illusion that a person was experiencing would only have a limited amount of choices, and the person would realize that something was amiss.