it can never rest satisfied with this outward view of natural things.
Brightness and freshness take possession of the mind when it is
crossed by the light of principles, showing the facts of Nature to be
organically connected.

Let us, then, inquire what this thing is that we have been generating,
reflecting, refracting and analyzing.

In doing this, we shall learn that the life of the experimental
philosopher is twofold. He lives, in his vocation, a life of the
senses, using his hands, eyes, and ears in his experiments: but such a
question as that now before us carries him beyond the margin of the
senses. He cannot consider, much less answer, the question, 'What is
light?' without transporting himself to a world which underlies the
sensible one, and out of which all optical phenomena spring. To
realise this subsensible world the mind must possess a certain
pictorial power. It must be able to form definite images of the things
which that world contains; and to say that, if such or such a state of
things exist in the subsensible world, then the phenomena of the
sensible one must, of necessity, grow out of this state of things.
Physical theories are thus formed, the truth of which is inferred from
their power to explain the known and to predict the unknown.

This conception of physical theory implies, as you perceive, the
exercise of the imagination--a word which seems to render many
respectable people, both in the ranks of science and out of them,
uncomfortable. That men in the ranks of science should feel thus is, I
think, a proof that they have suffered themselves to be misled by the
popular definition of a great faculty, instead of observing its
operation in their own minds. Without imagination we cannot take a