For the refuge in the Buddha to be genuine, it must be accompanied by a commitment to the Buddha as an incomparable teacher, as unexcelled and unsurpassed. Strictly speaking, the historical Buddha is not unique since there have been earlier Fully Enlightened Ones who have arisen in past epochs and there will be others who will arise in future epochs as well. But in any one world system it is impossible for a second Buddha to arise while the teaching of another Buddha is still extant, and thus in terms of human history we are justified in regarding the Buddha as a unique teacher, unequalled by any other spiritual teacher known to humanity. It is this readiness to recognize the Buddha as "the unsurpassed trainer of persons to be tamed, the teacher of gods and humans" that is the hallmark of an authentic act of taking refuge in the Buddha.
http://www.accesstoinsight.org/lib/auth ... ay_21.html
What does "world system" here stand for? Planet Earth? The Solar System? The Universe? Something else?