If nature has made any one thing less susceptible than all others of
exclusive property, it is the action of the thinking power called an
idea, which an individual may exclusively possess as long as he keeps
it to himself; but the moment it is divulged, it forces itself into
the possession of every one, and the receiver cannot dispossess
himself of it. Its peculiar character, too, is that no one possesses
the less, because every other possesses the whole of it. He who
receives an idea from me, receives instruction himself without
lessening mine; as he who lights his taper at mine, receives light
without darkening me. That ideas should freely spread from one to
another over the globe, for the moral and mutual instruction of man,
and improvement of his condition, seems to have been peculiarly and
benevolently designed by nature, when she made them, like fire,
expansible over all space, without lessening their density in any
point, and like the air in which we breathe, move, and have our
physical being, incapable of confinement or exclusive appropriation.

Blogged for future reference.