The pre-scientific conception of the dream which obtained among the ancients was, of course, in perfect keeping with their general conception of the universe, which was accustomed to project as an external reality that possessed reality only in the life of the psyche.

这里给出 that/which， 意思是让考生选择一个合适的关系代词，在前面 which 定语从句里面，又套着这个定语从句。这时，为避免重复使用相同的关系代词，最好选用 that。