The question of whether or not one can know whether one is dreaming has become a staple of philosophical discussion since Descartes wrote The Meditations in the 1600s. Engaging in philosophy for the first time, this can seem a bizarre question. However, Descartes’ reasoning for doubting the certainty that one is not dreaming is compelling. For Descartes, our ability to perceive reality cannot be guaranteed, since our senses can deceive us (Descartes, 1986). Thus, over the course of the first two Meditations, Descartes concludes that the only thing he is certain of is that there is some being that is “I”. He concludes that this “I”, however, may only be a mind (Descartes, 1986).

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Descartes reasons that even our perception of our bodies is a product of intellect. Therefore, the only thing he feels certain of is that there is a mind doing the thinking. There are two separate questions that arise from this. Firstly, can I know that I am awake? Secondly, can I know that my belief that I am not locked “inside a dream” is not itself a dream? This second question evokes the plot to a sci-fi film, and elicits imagery of being “a brain in a vat”, where everything that one perceives is illusory. The “brain in a vat” is a modern re-imagining of the demon argument, produced by Descartes. The “brain in a vat” idea originates with Putnam; and, according to Brueckner, is inspired The Matrix films (Brueckner). It is this second idea which will be the main focus of this essay – Descartes’ “demon argument”, or the “mind in a vat” argument.
This extreme form of scepticism, where one is merely a “brain in a vat” is surprisingly difficult to rule out with absolute certainty. However, the implications of this may be less profound than they initially appear. The notion that we do not have a true perception of the external world, because our sensory perceptions are being manipulated by a demon or we are a “mind in a vat”, may not actually have practical implications for how we live in the world. However, the discussion about whether we can know for certainty that we are not dreaming is not purely abstract and esoteric. There is an element of this that does pertain to a wider issue than merely dreaming. For instance, as Skirry explains, Descartes supposes that an evil demon may be deceiving him, and so as long as this supposition remains in place, there is no hope of gaining any absolutely certain knowledge (Skirry). If one cannot be sure that one is not being deceived by a demon, then one can have no absolutely certain knowledge about anything. However, as I will argue in this essay, concerning ourselves with  whether we lack true knowledge, because we are being manipulated by a demon does not help us to find solutions to the issues in the world which we believe we are living in.
The sceptical account for not knowing whether one is dreaming or not has two levels. First, our perception of what we are currently experiencing does not allow us to determine whether we are awake or dreaming. Dreams can have the same quality as waking experiences, and we can dream that we are awake. Therefore, the experience of being awake is not distinguishable from dreaming. Descartes provides the following example of this situation: “How often, asleep at night, am I convinced of just such familiar events – that I am here in my dressing-gown, sitting by the fire – when in fact I am lying undressed in bed!” (Descartes, 1986, p. 13). Given that in one’s dream, one’s perceptual experiences are not different from those when awake, it may be that I am dreaming that I am typing out this essay.
This sceptical consideration of the possibility of not having the knowledge that one is awake is not as profound or extreme as it first seems. It leaves intact the idea that there are two states: dreaming and awake. The problem is that when we think we are awake, we may be dreaming. It is, for this reason, that this essay will leave this discussion aside, and move on to the second level of scepticism explored by Descartes.
The second reason for doubting if we can know if we are dreaming takes scepticism to a deeper level. The sceptical account for doubting our ability to know if we are awake or if we are dreaming is summed-up by Blumenfeld and Blumenfeld as the problem of the possibility of being in a dream within a dream:
for all I know, I may be dreaming… now, then my belief that not all my experiences have been dreams is itself a belief held in a dream, and hence it may be mistaken. If I am dreaming now, then my recollection of having been awake in the past is merely a dreamed recollection and may have no connection whatever with reality. (Blumenfeld and Blumenfeld, 1978, pp. 243-244).
There are two ways of illustrating this dilemma. First is the illustration devised by Descartes, whereby one is being deceived by a demon. The second is the one favoured by sci-fi films, whereby one is merely a “brain in a vat”, and all that we think we are experiencing has no relation to external reality. 
This second level of scepticism speculates that all our experiences may be locked within a dream, including our experiences of waking and dreaming. Given the time period in which he was writing, Descartes invokes superstitious and supernatural ideas of a God or a demon to illustrate this. Descartes imagines that there may be:
some malicious demon of the utmost power and cunning [that] has employed all his energies in order to deceive me. I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgement. I shall consider myself as not having hands or eyes, or flesh, or-blood or senses, but as falsely believing that I have all these thing (Descartes, 1986, p. 15).
The modern sci-fi parallel is that I am actually, merely “a brain in vat”, probably millions of miles away, on some distant planet. This is the view that everything we experience of the external world is a deception. This modern, scientific alternative allows the modern reader to see Descartes problem more clearly, and prevents us dismissing it is an anachronism from the time of superstition.
In the Second Meditation, Descartes convinces himself that because he is thinking, he does actually exist. Hence the famous phrase: Cogito, ergo sum or “I think, therefore I am” (Descartes, 1986, p. 17) ). This is important, as it does set a limit to scepticism, since Descartes’ conclusion is that “even if I am being deceived by an evil demon, I must exist in order to be deceived at all” (Skirry). The fact that I think is proof that I am at least a mind. However, this does not provide proof that I am also a body. Descartes poses to himself the question: “what am I to say about this mind, or about myself?” (Descartes, 1986, p. 22). But he then tells the reader, “so far, remember, I am not admitting that there is anything else in me except a mind” (Descartes, 1986, p. 22). Descartes famous phrase cogito, ergo sum is part of the philosophical canon because it is Descartes’ demonstration that there are limits to scepticism – I think; therefore, I am a mind. However, the knowledge that I am thinking does not, in itself, rule out the possibility that I am merely a mind, i.e. that I am locked in a dream within a dream, where I am deceived into thinking that I have two states of existing: one, being awake; the other being dreaming.
At the beginning of this essay, I said that, engaging in philosophy for the first time, the question, “can we know we are not dreaming?”, can seem a very bizarre question. This can be seen in Blumenfeld and Blumenfeld’s paper, when they show that “a frequent charge against scepticism is that it shows that we cannot have knowledge only by adopting an implausibly strong definition of knowledge” (Blumenfeld and Blumenfeld, 1978, p. 249). Intuitively, the idea that “I” (whatever I am in this case) am merely “a mind in a vat” is implausible. This is why the question, “can we know we are not dreaming?”, seems bizarre. It may not be possible to know that we are not dreaming. However, this requires the construction of a rather implausible hypothesis. In other words, only by invoking something that seems implausible can the question “can we know we are not dreaming?” be made.
However, to dismiss Descartes and the sceptics’ argument on these grounds is rather weak. Dismissing the demon argument on the basis that it is implausible does not falsify it. This is just an argument of probability. The argument that it seems more probable that I am not dreaming, and I do experience an external world is not sufficiently sound, philosophically, to end the argument. There is a need to produce a more satisfying philosophical explanation.  Blumenfeld and Blumenfeld argue that it is not possible to justify empirical claims on the basis of probability (Blumenfeld and Blumenfeld, 1978). Therefore, they argue that to maintain the argument of an external world, and rule out the demon scenario, the hypothesis of an external world needs to be epistemically superior to the hypothesis of a world constructed by a demon (Blumenfeld and Blumenfeld, 1978).. However, Blumenfeld and Blumenfeld are not convinced that the hypothesis of an external world is epistemically superior. They argue:
One might think that this could be argued on grounds of the greater simplicity of the external-world hypothesis. But it is hard to see in what respect the external-world hypothesis is simpler than that of the demon. The latter is committed to the existence of the demon (a spirit) with the means of and a motive for producing sense experiences, to a mind in which these experiences are produced, and to the sense experiences themselves. The external-world hypothesis, on the other hand, is committed to all of the above, except the existence of the demon. But it is committed, in addition, to a physical world with the capability of producing sense experience. So, it is hard to see how the external-world hypothesis is simpler. (Blumenfeld and Blumenfeld, 1978, p. 250).
Therefore, it is surprisingly difficult to rule out the idea that “I” am “a mind in a vat”, and that all my experiences of the external world are based on a deception to my sensory perception.
However, the implications of this may not be as profound as they initially appear. Firstly, the implications that all our experiences of an external world are based on illusion would only come into existence if the illusion is broken. If there is a demon creating sensory experiences for me, or I am actually just “a brain in a vat”, the implication of this would only occur when I became aware of my “real” existence, and of the illusion and deception. Secondly, unless we become aware that all our past experiences, including those of being awake and of dreaming, are part of a dream, we are no better able to deal with the dilemmas of this world than we are currently. It is hard to see what the practical implications of this theory are. Or, more specifically and more importantly, how they can help us. For example, it isn’t going to work to tell a Syrian refugee, “don’t worry, go back to Syria, because the war isn’t real. We are actually ‘brains in a vat’, on another planet, many millions of miles away.” It may sound as though I am being facetious. However, the point is a serious one. The question: “can you know that you are not dreaming?” may be a valid one – it might be surprisingly difficult to prove that I not “a brain in a vat”. However, it is not a very helpful question to be concerning ourselves with.
In conclusion, demonstrating that our sensory experiences are not the trickery of a malicious demon proves unfruitful. Trying to satisfactorily refute the idea fails to recognise that the implications of this would only matter if we found out that in the “real” world, we were just “minds in a vat”. Meanwhile, there are practical concerns that require our thought, such as the Syrian refugee problem. The kinds of questions that scepticism is concerned with do not help us to deal with these practical issues. However, it does make us wonder if these “practical” issues are real. Descartes’ hypothesis makes us ponder the possibility that the Syrian refugee crisis is not real, and is part of the deceptions of a demon. However, this kind of thinking does not help us to respond to the things that we think are important.
Bibliography
Blumenfeld, D. and Blumenfeld, J.B. (1978) “Can I Know that I am not Dreaming?” in Hooker, M (ed.), Descartes: Critical and Interpretative Essays. John Hopkins, Baltimore, pp. 234-253
Brueckner, T. (Retrieved October 15, 2015). “Skepticism and Content Externalism”. Stanford Encyclopedia of Philosophy, Available from: http://plato.stanford.edu/entries/skepticism-content-externalism/#2
Descartes, R. (1986) “First Meditation”, in Cottingham, J (trans.) Meditations on First Philosophy: With Selections from the Objections and Replies. Cambridge University Press, Cambridge, pp. 12-15
Descartes, R. (1986) “Second Meditation”, in Cottingham, J (trans.) Meditations on First Philosophy: With Selections from the Objections and Replies. Cambridge University Press, Cambridge, pp. 16-23
Descartes, R. (1986) “Objections and Replies [Selections]” in Cottingham, J (trans.) Meditations on First Philosophy: With Selections from the Objections and Replies. Cambridge University Press, Cambridge, pp. 63-67
Skirry, J. (Retrieved October 6, 2015), “René Descartes (1596—1650)”, Internet Encyclopedia of Philosophy, Available from:  http://www.iep.utm.edu/descarte/