This statement means that anything that you are thinking, or anything that is on your mind, is there because someone or something did or said something to put such a thought there in the first place.  A thought cannot and does not appear suddenly or without reason, and is never something comepletely new/alien.

Like this idea on the origin of thoughts, a research project is not something you write out of nowhere just because you had an inspiration to.  Writing a research paper can require vast amounts of research, and even after all that research you are only basing and building onto a certain topic, already filled with tons of information alien to your own, with a little bit of your own carefully formed opinion.