[I thought I'd used part of this one before, but Google doesn't think so (and my greppable home archive of my journal entries was on the hard disk that died and I haven't recreated it yet).]
From the Quotation of the day mailing list, 2007-03-16:
"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." -- Thomas Jefferson, from a letter written to Isaac McPherson in 1813.
(submitted to the mailing list by Mike Krawchuk)
[In addition to being Thomas Jefferson's birthday, the calendar file tells me this is the Laotian new year.]