In his book Is There Life on Other Worlds, Poul Anderson makes a point about how long it would take to explore the galaxy. The "isolation" argument is usually used to explain why we have not been visited by extra-terrestrial life, i.e. it would take so long that this can be taken as a reason that we have not encountered any aliens so far (to our knowledge). However, the argument contains a flaw, which is that the exploration, or the growth of knowledge about explored systems, is linear.