ented to explain._ Thus, in the discovery of Neptune, after the
existence of such a planet outside the orbit of Uranus had been
conjectured (to account for the movements of the latter), the place in
the heavens which such a body should occupy at a certain time was
calculated, and there by means of the telescope it was actually seen.

Agents, however, are assumed and reasoned upon very successfully which,
by their nature, never can be objects of perception: such are the atoms
of Chemistry and the ether of Optics. But the severer methodologists
regard them with suspicion: Mill was never completely convinced about
the ether; the defining of which has been found very difficult. He was
willing, however, to make the most of the evidence that has been adduced
as indicating a certain property of it distinct from those by which it
transmits radiation, namely, mechanical inertia, whereby it has been
supposed to retard the career of the heavenly bodies, as shown
especially by the history of Encke's comet. This comet returned sooner
than it should, as calculated from the usual data; the difference was
ascribed to the influence of a resisting medium in reducing the extent
of its orbit; and such a medium may be the ether. If this conjecture
(now of less credit) should gain acceptance, the ether might be regarded
as a _vera causa_ (that is, a condition whose existence may be proved
independently of the phenomena it was intended to explain), in spite of
its being excluded by its nature from the sphere of direct perception.
However, science is not a way of perceiving things, but essentially a
way of thinking about them. It starts, indeed, from perception and
returns to it, and its thinking is controlled by the analogies of
perception. Atoms and ether are thought about as if they could be seen
or felt, not as noumena; and if still successful in connecting and
explaining perceptions, and free from contradiction, they will stand as
hypotheses on that ground.

On the other hand, a great many agents, once assumed in order to explain
phenomena, have since been explained away. Of course, a _fact_ can never
be 'explained away': the phrase is properly applicable to the fate of
erroneous hypotheses, when, not only are they disproved, but others are
established in their places. Of the Aristotelian spheres, which were
supposed to support and translate sun, moon and planets, no trace has
ever been found: they would have been very much in the way of