In his Meditations on First Philosophy (1641), René Descartes (1596-1650) set out to establish a firm foundation for the sciences. In attempting to do so he succeeded, via two of the most famous sceptical arguments in philosophy, only in confirming how little we actually know for certain.

In the Medieval world [and indeed still to this day in some dark corners] the best way of acquiring knowledge was via Revelation or Authority. However, Descartes was writing at a time of scientific revolution when many doctrines which had hitherto been accepted as certain were being overturned and, as such, he was struck by the instability and unreliability of scientific ‘knowledge’.

In his First Meditation, Descartes aims to sweep away all of his previously held opinions and start afresh. His ‘method of doubt’ entails that if anything can be doubted, however slightly, then we are to treat it as if it is manifestly false and reject it outright. It is not, however, necessary that we subject each and every one of our opinions to this hyperbolic (exaggerated) doubt, as this would be a Sisyphean task. Rather, Descartes aims to test the ‘foundations’ of what we claim to know – ‘as the removal from below of the foundation necessarily involves the downfall of the whole edifice’.

Descartes claims that our knowledge comes either from the senses or from reason, and in order to subject these foundations to his method of doubt he unleashes two powerful arguments: the dream and demon hypotheses. Firstly, let us consider the Dream Hypothesis. What Descartes hopes to establish via this argument is that, given any particular experience, we can never know that that experience is not a dream. As such, we have reason to doubt the senses and therefore should treat all beliefs that we receive from them as manifestly false. [Note that all Descartes requires here is the universal possibility of illusion (that for any experience we have, that experience may be a dream) not the possibility of universal illusion (that we may be dreaming all the time).]

Descartes believes, however, that some beliefs are immune from the dream hypothesis – namely, those that we gain not from experience but from reason alone – ‘for whether I am awake or dreaming, it remains true that two and three make five, and that a square has but four sides’. In order to subject this realm to the method of doubt Descartes conjures the second of his sceptical arguments: the demon hypothesis.

I will suppose … that some malignant demon, who is at once exceedingly potent and deceitful, has employed all his artifice to deceive me; I will suppose that the sky, the air, the earth, colors, figures, sounds, and all external things, are nothing better than the illusions of dreams.

In case you find the introduction of a malevolent genie too much to bear, Hilary Putnam (Reason, Truth, and History, 1981) updates the argument by asking you to consider the possibility you are a brain in a vat and that your experiences are just electronic signals being sent to your disembodied brain by an evil scientist.  [‘Utter nonsense!’ you exclaim, but that’s exactly what a brain in a vat would say.]  This thought experiment also forms the plot of the Hollywood blockbuster The Matrix.

By the end of the First Meditation Descartes has done a terrific demolition job.  By applying his method of exaggerated doubt to the foundations of knowledge the entire edifice has indeed crumbled.  At the outset of Meditation II, he states that:

The Meditation of yesterday has filled my mind with so many doubts, that it is no longer in my power to forget them… I suppose, accordingly, that all the things which I see are false (fictitious); I believe that none of those objects which my fallacious memory represents ever existed; I suppose that I possess no senses; I believe that body, figure, extension, motion, and place are merely fictions of my mind. What is there, then, that can be esteemed true? Perhaps this only, that there is absolutely nothing certain.

In order to progress, Descartes must establish one thing that is certain and indubitable.  This eventually leads him to the cogito: ‘this proposition I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind’.  This one truth, believes Descartes, is immune from all doubt, for even a malignant demon cannot deceive me into thinking that I exist if I do not; if I am being deceived, then I exist.