Descartes begins with the senses. He says: "Of course, whatever I have
so far accepted as supremely true I have either learned from the senses or
through the senses." So, why would one reject beliefs which come from
such a faculty? He continues: But I have occasionally caught the senses
deceiving me, and it is never prudent to trust those who have cheated us
even once." He then goes on to say: "...my senses may deceive me about
what is small or far away..." Can you think of an example where the
senses deceive you in some way about something far away? How about the
size of the sun or moon or stars? These large bodies all appear
relatively small because they are at a great distance from us. Or what
about the shape of towers. A square tower at a distance may appear round.
Descartes gives no examples here, but we have good reason to think that
these are the sorts of things which he has in mind. (This is because he
does give these examples later -- in Med. VI.)
Now we begin to see how the Method of Doubt works. If our first
skeptical hypothesis is that we may be deceived by our senses about things
seen at a distance (or they very small), it is clear that there are a
whole range of things which this hypothesis does not touch.
particular, since it deals with things far away, it does not call into
question things close at hand. In the sentence quoted above about the
senses deceiving him about things far away, he continues: "...there may
still be other things which I take in by the senses but which I cannot
possibly doubt -- like I am here, sitting before the fire, wearing a
dressing gown, touching this paper." So there are things which one can
doubt on the first skeptical hypothesis, and things which one cannot
doubt. The skeptical hypothesis, as I noted earlier, works live a sieve.
It strains out one class of beliefs and leaves others go. It is also
worth noting that Descartes has neatly partioned truths known by the
senses into two classes.
|Sitting by the
|Claims about things known at a distance|
These claims are often and regularly false,
sun is the size of my thumbnail"
|Claims about things perceived
close at hand|
These claims are often and
regularly true, e.g.
"I am sitting
by the fire"
|Knowledge claims from the senses|
This partitioning of the classes of truths derived from each faculty is going to be of great utility to him both in going down and in the ascent back up. The parts that he has trouble doubting will be made secure from below. So, for example, when Descartes partitions the truths known from the imagination into dream images and mathematical images, it turns out that when the truths of reason and hence mathematics are made secure by the proof for the existence of God, this in turn secures the truth of the mathematical images that are derived from those known by reason alone.
In order to doubt the beliefs which cannot be doubted on this first
skeptical hypothesis, Descartes needs a stronger skeptical hypothesis.
But it seems much more difficult to doubt claims about the senses when we
are considering things close at hand than things at a distance. Descartes
cleverly overcomes this difficulty by basing his new and stronger
skeptical hypothes not in the senses but in the imagination. He can do
this because some images might come either from the senses or from the
imagination -- that is there is a kind of causal overlap between these