7.4.1 Global Skepticism

Global skepticism is a view that questions the possibility of all knowledge. To make their case, global skeptics point to the lack of the possibility of certainty in our beliefs. Because we cannot know that our beliefs are true, we cannot know in general. Usually, global skepticism attempts to undermine the possibility of forming justified beliefs. Global skeptics target all beliefs, or all beliefs about the external world (which amounts to most beliefs). Most beliefs tacitly or explicitly assume the existence of an external world. When I have the experience of seeing a bird in a tree and think, “There is a bird in that tree,” I assume that there is an actually existing physical bird in an actually existing physical tree in an actually existing real world outside of me. There is means “there exists.” I believe the bird, tree, and world all exist independently of my thoughts. The global skeptic questions beliefs such as these.

The Dream Argument

How many times have you realized that you were dreaming while you were dreaming? Most people believe that whatever they are dreaming is real during the dream. Indeed, the fact that people think dreams are real while dreaming is what makes nightmares so terrible. If you knew the content of a nightmare was a dream, then it would not be nearly as scary. Zhuang Zhou (c. 369–286 BCE) was a Chinese Taoist philosopher who argued that for all we know, we could currently be dreaming while thinking we are awake. Imagine dreaming that you are a butterfly, happily flitting about on flowers. When you wake, how can you determine whether you have just woken from dreaming you are a butterfly or you are a butterfly who has just started dreaming that you are human? Zhuang Zhou explains:

While he is dreaming he does not know it is a dream, and in his dream he may even try to interpret a dream. Only after he wakes does he know it was a dream. And someday there will be a great awakening when we know that this is all a great dream. Yet the stupid believe they are awake, busily and brightly assuming they understand things, calling this man ruler, that one herdsman—how dense! Confucius and you are both dreaming! And when I say you are dreaming, I am dreaming, too. (Zhuangzi 2003, 43)

Zhaung Zhou puts forward the possibility that all of what we take to be conscious experience is actually a dream. And if we are dreaming, then all our beliefs about the external world are false because those beliefs take for granted that our current experience is real.

The Evil Demon Argument

Nearly two millennia after Zhuang Zhou, René Descartes also proposed a dream hypothesis. Descartes argued that because dreams often incorporate experiences we have in real life, it is impossible to distinguish between dreaming and waking life (Descartes 2008). But Descartes eventually concludes that even if he could be dreaming, there are still some beliefs he can know, specifically arithmetic. Even in dreams, 1 + 1 = 2, and a square will always have four sides. And so, Descartes devises an even stronger skeptical hypothesis: what if we are being tricked by an evil demon?

Descartes’s evil demon is powerful. It can make you believe things, and it can trick you by controlling your experience. The evil demon can make you believe you are currently eating a sandwich by directly feeding you the sensory experience of eating a sandwich (the sight, the smells, the taste, the feel). Under this scenario, you cannot tell the difference between actually eating a sandwich and merely believing you are eating one because the evil demon is tricking you. If we cannot reliably tell the difference between experiences caused by reality and experiences caused by an evil demon, then we cannot know anything. We can represent Descartes’s argument as follows:

1. If I cannot rule out the possibility that an evil demon is tricking me, then I do not have any knowledge of the external world.
2. I cannot rule out the possibility that an evil demon is tricking me.
3. Therefore, I do not have knowledge of the external world.

Why does Descartes claim we can’t have knowledge if we cannot rule out the evil demon hypothesis? If an evil demon is tricking us, then all our beliefs are wrong. And if we cannot rule out the possibility that we are wrong, then we are not justified. And if we are not justified in our beliefs, then we cannot have knowledge of them.

Putnam’s Brain in a Vat

If you don’t like evil demons, then consider a more modern version of a skeptical hypothesis: the “brain in a vat” conceived of by American philosopher and mathematician Hilary Putnam (1926–2016). Imagine that while you were asleep last night, a group of scientists kidnapped you and took you to their lab. There, they surgically removed your brain and placed it in a vat of nutrients. The scientists then hooked up your brain to a sophisticated new computer system. They were able to download your memories so as to create new experiences. The result is a seamless experience of consciousness between yesterday and today. When you woke this morning, your life seemed to proceed without disruption. Can you prove that you are not a brain in a vat? No, you cannot. The scenario stipulates that your experience will seem exactly the same whether you are a brain in a vat or not. Other, similar skeptical scenarios are easy to come up with. Consider the possibility that you are caught in a virtual reality world or that you are trapped in the Matrix.

General Structure of Global Skeptical Arguments

Skeptical hypotheses and the arguments that they inspire all have a similar structure:

1. If I cannot rule out the possibility of SH, then I cannot be justified in believing that P.
2. I cannot rule out the possibility of SH.
3. Therefore, I cannot be justified in believing that P.

SH is a skeptical hypothesis. P is any proposition about the external world. Premise 1 is the skeptic’s challenge—that you must rule out skeptical hypotheses. Premise 2 relies on limitations within your perspective. The skeptic claims that you can rule out the possibility of whatever skeptical hypothesis is at hand only if you are able to construct an argument that defeats that hypothesis using the evidence you have (and a priori knowledge). As demonstrated, this is difficult to do. The nature of the skeptical hypotheses used for global skepticism limits your evidence to the contents of your thoughts. What you take to be evidence of the external world (that you perceive things that seem to be separate from yourself) is effectively neutralized by the possibility of a skeptical hypothesis.