c CUNYB/Clarke December, :
Retreat and Defence (–) Yo uknow very well that the appearance of celestial movements results equally certainly
from the assumption that the earth is at rest as from the assumption that it is in motion.
Therefore, the experience of this appearance is not sufficient to prove which of these
two causes just mentioned is the true cause....There is nothing easier than to adjust
some cause to a given effect, and you know that this is familiar to astronomers, who
bymeans of different hypotheses, of circles and ellipses, come to the same conclusion.
The same thing is very well known in yourGeometry.However, inorder to prove that
the cause of an effect is the true and unique cause, it is necessary to prove at least that
such an effect could not be produced by any other cause. (i.,)Morin also raised queries about many details of the Cartesian theory of
light, and even about the fundamental model of light that underpinned the
Dioptrics.Inthe passage just quoted, however, he was focusing primarily
onthe logic of confirmation. Even if Descartes’ hypotheses were readily
intelligible, and even if one could construct a plausible account of how they
cause the various optical phenomena that they are supposed to explain,
it remained an open question whether this theory was correct or whether
some alternative hypothesis might work equally well.
Descartes took almost five months to prepare a lengthy reply to this
challenge. He may have delayed sending his response because he had
planned to avail of Morin’s permission to publish his objections, together
with the replies, without showing them to their author.Once that plan
was abandoned, he sent detailed, sequentially numbered replies to each
of Morin’s objections. Descartes had to accept the general point, that the
observed phenomena about light could have been explained as easily by an
alternative theory. ‘If light can be imagined in some other way by which
onecan explain all of its properties that are known by experience, one will
see that everything that I have demonstrated about refractions, vision, etc.
can be deduced from it as well as from the way that I proposed’ (ii.).
Likewise, he could hardly have disagreed that it is always easy to imagine
a specific cause that is tailor-made to explain a given effect. ‘There are
truly many effects to which it is easy to adjust different causes’ (ii.).
Having conceded this much, however, he went on to defend both the logic
of his scientific method and all the particular explanations that Morin had
challenged.
There was no way to avoid the appearance of circularity – and there
is still none, more than three centuries later – that is involved in making
an observation (for example, observing the colours of the rainbow), imag-
ining some cause that could explain this phenomenon, and then using