1 The Birth of Science - MSRI

6.3 Saving the Phainomena 153
If phainomena, because of their immediate evidence, are singled out as
the only indubitable epistemological data, they are the best candidate for
a departure point in the construction of scientific theories. But they can page 197
only be the departure point in a heuristic, not a logical, sense; they must
then be explained by a theory logically based on “hypotheses” that are
not directly verifiable, and in this theory they play the role of effects. On
this point there is an important testimonium in the medical work of the
Anonymus londinensis:
. . . as Herophilus observes, saying “let the phainomena be described
first, even if they do not come first.” 20
In the case of optics — the theory of sight — the only certain data from
which one can start are visual perceptions. One of the propositions that
Euclid demonstrates is:
If several magnitudes move [in the same direction] each at its own
speed, and the eye also moves in that direction, those magnitudes
that move at the same velocity as the eye appear stationary, those
that move more slowly appear to fall behind, and those that move
faster appear to advance. 21
The phainomena, which heuristically speaking represent the starting
point, are here deduced from (not directly verifiable) statements about the
state of motion of the observer and the objects observed. Euclid’s proposition
has a great deal of methodological interest, recognizing as it does
that, at least in the case of visual perception, the phainomena do not speak
about the object directly, but only about a relationship between object and
observer.
A connection between Euclid’s proposition and the astronomical problems
of his time is transparent: retrogressing planets were very soon to be
seen as a case of bodies that seem to move backward because their motion
is slower than the observer’s.
We now see clearly what essential requirement the “hypotheses” of a
theory must satisfy: they need not be directly verifiable, they may even be
surprising at first sight, but the important thing is that they must allow
the logical deduction of the phainomena: in the case of astronomy, the
observed motion of heavenly bodies. The Aristarchan “hypothesis” that page 198
the sun was stationary and the earth had movements of rotation and revolution
certainly appeared strange and remote from intuition, but (and this
is the crux!) it allowed their inventor to “save the phainomena” (
20 P. Londinensis 137 = [von Staden: H], text 50a, 3–4.
21 Euclid, Optics, Prop. 51.
Revision: 1.7 Date: 2002/09/14 23:17:37