Thanks a lot.
It seems that I was not able to get my idea across. What I was asking about is what he has seen in the lives of his mentors. Has he seen the principle or he has seen the (effect) of the principle in the lives of his mentors. I know that the principle has an effect on the author's life. But my question is about the lives of his mentors. How is the principle seen? In my opinion, it cannot be seen. What is seen (implied) in the sentence is the effect. So the sentence could be modified a bit as follows:
As I have seen (the effect of) this principle of long-term thinking in the lives of my mentors.
I hope I was able to make my point clear.