The theme of the entire lesson you are reading is how the world we live in is a reflection of our predominant thought patterns i.e. mental attitude.

Throughout this lesson Mr Haanel compares and contrasts the world within to the world without.

He is basically saying that once you recognize that everything that is without you is just a reflection of what is within you, you then have access to the true source of all power in your life.

And in that paragraph that you quoted, he is effectively saying that your thoughts interact with the Universal Mind to create the circumstances of your life which are brought about by that Universal Mind.

And he is also saying that your interaction with this Universal Mind is a subjective thing (i.e. personal to you) and not an objective thing (i.e. something that a scientist might try and measure) and this accounts for the many differing conditions and experiences that people go through in their lives.

In a nutshell, I believe he is saying that the more you make an effort to think thoughts that are in harmony with the Universal Mind, the better your life will be in every way.

Does that help a bit?

If not, please clarify exactly which bit (or word) is confusing you and we can try to clear up the confusion.

This book is around 100 years old, and the old style of language can get tricky to understand sometimes so don't feel bad if some of it seems hard to grasp first time around.

"...but the law of action and reaction does not rule within a single human, but within all there is. It is not a neutral, sober thing, but rather a development that affects the perspective of the single human, who can be watched as he undergoes the outcomes [of his(?) actions] in a never-ending diversity of life situations and learning."

There, almost no word reused. ;-)

OK, I'll take a stab at it.

the principle of causation does not obtain in the individual but in the Universal Mind

The energy of creation that allows you to create things in your life does not come from you, but from God. God creates everything through you.

It is not an objective faculty but a subjective process

We see the world through our own perceptual filters. If you have a specific belief, you will perceive the world in such a way that your belief is validated.

Our mind is like a camera. It focuses on what we choose to focus on based on our beliefs. If we believe that people are generally unfriendly, we will focus on those people that are unfriendly. We conclude that all people are unfriendly, because this is generally what we see.

When people say they "create their world," this is what they mean.

the results are seen in an infinite variety of conditions and experiences

As a conduit for God's power and grace, that power can manifest itself in as many ways as free will allows you to choose.