An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

308
THE THEORY OE INDUCTION
bodies by gravitation ; and that direction is always in a
straight
line towards the earth s centre.
The second rule implies not only that the hypothesis
must be clearly and distinctly conceived in itself; it
must also be conceived after the analogy of something
in our experience. To assume something utterly un
like all that we are previously acquainted with,
is to
assume what can be neither proved nor disproved, for
we could not draw any conclusions from it. Even the
hypothesis of an absolutely solid and elastic something,
to explain the phenomena of light,
is not of this kind.
Jevons says truly that if this
"
"
luminiferous ether were
wholly different from everything else known to us, we
should in vain try to reason about it. "We must
apply to it at least the laws of motion that is, we
must so far liken it to matter. And as, when applying
those laws to the elastic medium air, we are able to
infer many of the phenomena of sound, so by arguing
in a similar manner concerning ether we are able to
infer many of the phenomena of light.
All that we do
is to take an elastic substance, increase its elasticity im
mensely, and denude it of gravity and some other prop
erties of matter ; but we must retain sufficient likeness
to matter to allow of deductive calculations."
Newton did not use the word hypothesis as we now
use it. He used it to signify just such unprovable
assumptions as are excluded by
Hence he said hypotheses non fingo,
this second rule.
"
I do not imagine
hypotheses." The word is still occasionally used by
scientific writers in this sense.
The third condition is one that we have already
illustrated. The consequences of the hypothesis must
be deduced as rigorously as possible, and then com
pared with the results of accurate observation. The