I was reading An Essay Concerning Human Understanding by John Locke and came across a really strange sentece. Here is the sentence:
We shall not have much reason to complain of the narrowness of our minds, if we will but employ them about what may be of use to us; for of that they are very capable.
I didn't understand two points:
Firstly, after "If we will.." why there is a "but"? What does it mean but here? Is it different usage?
Secondly, I have understood nothing from the part "for of that they are very capable" both in terms of grammar and meaning. Because ıt looks interesting putting "of" after "for". Also, where is "for" linked to?
Finally, if you summary the general meaning it would be useful for me.
Thanks for your time
By the way, for these who want to see the context of the text I am putting whole the paragraph:
Men may find matter sufficient to busy their heads, and employ their hands with variety, delight, and satisfaction, if they will not boldly quarrel with their own constitution, and throw away the blessings their hands are filled with, because they are not big enough to grasp everything. We shall not have much reason to complain of the narrowness of our minds, if we will but employ them about what may be of use to us; for of that they are very capable. And it will be an unpardonable, as well as childish peevishness, if we undervalue the advantages of our knowledge, and neglect to improve it to the ends for which it was given us, because there are some things that are set out of the reach of it.
(Because of being understood easily by everyone I've put the whole paragraph and make the sentence that I didn't understand bold):