we are not conscious of the extent ------work provides the p

extent是程度;限度;范围的意思
经常与to连用,例如：To a certain extent,从某种程度上说
在这里,也是同样的道理,而which则是与后面的句子相连,作为疑问词来补充成分,which work provides the psychological satisfaction that can make the difference between a full and an empty life.这实际上是一个完整的句子.
全句的意思是：我们并不太清楚要区分一个有意义的和空虚的人生需要多大程度的精神满足.