that the pulmonary artery carries dark blood and the pulmonary vein
carries bright blood, departing in this respect from the common rule,
for the division of labour between the veins and the arteries. But every
one knows how we seek naturally to combine the pieces of our knowledge
together, to bring them under general rules, to relate them to
principles; and how unsatisfactory and tiresome it would be to go on
forever learning lists of exceptions, or accumulating items of fact
which must stand isolated.

Well, that same need of relating our knowledge, which operates here
within the sphere of our knowledge itself, we shall find operating,
also, outside that sphere. We experience, as we go on learning and
knowing,--the vast majority of us experience,--the need of relating what
we have learned and known to the sense which we have in us for conduct,
to the sense which we have in us for beauty.

A certain Greek prophetess of Mantineia in Arcadia, Diotima by name,
once explained to the philosopher Socrates that love, and impulse, and
bent of all kinds, is, in fact, nothing else but the desire in men that
good should forever be present to them. This desire for good, Diotima
assured Socrates, is our fundamental desire, of which fundamental desire
every impulse in us is only some one particular form. And therefore this
fundamental desire it is, I suppose,--this desire in men that good
should be forever present to them,--which acts in us when we feel the
impulse for relating our knowledge to our sense for conduct and to our
sense for beauty. At any rate, with men in general the instinct exists.
Such is human nature. And the instinct, it will be admitted, is
innocent, and human nature is preserved by our following the lead of its
innocent instincts. Therefore, in seeking to gratify this instinct in
question, we are following the instinct of self-preservation in