The last part,
It is but feeble if it does not see so far as to know this.
could be rephrased as:
Reason is weak if it not does realize that there is an infinity of things which are beyond it.
By the way, if you find the original French sentence and use Google Translate, the result is easy to understand and, surprisingly, pretty accurate (you just need to change she to it).
The last step of reason is to recognize that there is an infinity of things that surpass it; [it] is only weak, if [it] does not go so far as to know that.