"We are like sailors who on the open sea must reconstruct their ship but are never able to start afresh from the bottom. Where a beam is taken away a new one must at once be put there, and for this the rest of the ship is used as support. In this way, by using the old beams and driftwood the ship can be shaped entirely anew, but only by gradual reconstruction." - Otto Neurath
Neurath was using the metaphor to explain his view of how human knowledge is constructed, but it was popularized by W.V. Quine who used it to help describe his view of naturalized epistemology, in which he sought to make epistemology a subset of psychology.
I was going to explain further, but while searching for Quine's original essay, "Epistemology Naturalized", I came across this post from Lindsay Beyerstein who has already said it better than I think I can.