Even scientific experiments depend on the senses.
We mix two chemicals and observe what
results, or we let some ball bearings drop
and observe how they behave.
Descartes would like this heavy reliance on
sensory observation to stop.
He admits that for some knowledge
the senses are required.
For example, I could not know what
books were on my desk if I did
not use my eyes to check.
However, he does not believe that we need sensory input
when doing science.
In fact, he is convinced that the senses only
mislead us in scientific endeavors.
Science, he feels, should proceed strictly by
tracing logical connections between ideas of the intellect and
not by observation.
The senses do not even practically provide us
with the ideas that we use in this reasoning.
We are born with them already in our minds.
Descartes, therefore, begins the Principles with two skeptical
worries meant to undermine our faith in the senses.
He points out, first, that our senses systematically mislead us.
For example, when we view a straight stick through water, it
looks bent; when we view things from a distance we tend to see
them as much smaller than they are, or even as a different shape.
Not only are the senses periodically unreliable, however,
they are also constantly and stubbornly unbelievable.
When we sleep we often have sensations indistinguishable
from those that we have when we are awake.
We admit that those dreaming sensations do
not correspond to reality, so why are we any more certain of
our waking sensations?
