Contemplation: Why is this?

After reviewing the guidelines for practice, take the following as your contemplation:

Buddha said:  Subhuti, all the bodhisattva heroes should discipline their thoughts as follows:  All living creatures of whatever class, born from eggs, from wombs, from moisture, or by transformation, whether with form or without form, whether in a state of thinking or exempt from thought necessity, or wholly beyond all thought realms--all these are caused by me to attain unbounded liberation nirvana.  Yet when vast, uncountable, immeasurable numbers of  beings have thus been liberated, verily no being has been liberated.  Why is this, Subhuti?

from the Diamond Sutra, chap. 3, trans. A.F. Price and Wong Mou-Lan