下面这篇英语是新概念第四册第44课中的最后两段,请问最后一段为什么用一般过去时态而不用一般现在时?

下面这篇英语是新概念第四册第44课中的最后两段,请问最后一段为什么用一般过去时态而不用一般现在时?
The study of custom can be profitable only after certain preliminary
propositions have been accepted,and some of these propositions have been
violently opposed.In the first place,any scientific study requires that there be no
preferential weighting of one or another of the items in the series it selects for its
consideration.In all the less controversial fields,like the study of cacti or termites
or the nature of nebulae,the necessary method of study is to group the relevant
material and to take note of all possible variant forms and conditions.In this way,
we have learned all that we know of the laws of astronomy,or of the habits of the
social insects,let us say.It is only in the study of man himself that the major social
sciences have substituted the study of one local variation,that of Western
civilization.
Anthropology was by definition impossible,as long as these distinctions
between ourselves and the primitive,ourselves and the barbarian,ourselves and the
pagan,held sway over people's minds.It was necessary first to arrive at that
degree of sophistication where we no longer set our own belief against our
neighbour's superstition.It was necessary to recognize that these institutions which
are based on the same premises,let us say the supernatural,must be considered
together,our own among the rest.

一般过去时表示“过去的、与现在无关的（或曾经存在、现在已不存在的）的情况.比如Your child was a good student.这句话,可以指你的已经长大的孩子,在小时候是个好学生.也可以指你的小孩最近变得很调皮.上面这段话,第一句可以译为“人类学从定义上来说曾经是不可能的”,也就是说在过去那种条件下是不可能进行人类学研究的.