This is my personal opinion:

Reading that statement would be good for a scientific close-mind person but it can be taken in many ways depending on the person. As an investigator, I dont think that way. Hated and goodness isnt defined by what we have, but what we have done. Our reactions define us.

For example, certain religious organizations may agree with the statement. Some might not.

To me, lightness and darkness is something that was not created by people, but we are born with both.

Vernocia, if you look back to religion for example, God has given us ALL free will. We have the power to do what is RIGHT and what is WRONG, just as we are raised by parents/guardians to know right from wrong. It is in your personal morales, beliefs,etc that you choose to do either of the two. The "devil", Lucifer, Satan, or "the serpent" however you choose to call it, is out there in the world constantly trying to throw you off course of the path of righteousness that will ultimately bring one to God on your final judgement, or if give into the wrong,will bring you to the eternal enferno.

When we are born, because of the sins and WRONG DOINGS of Adam and Eve, we are all born into the darkness of original sin. When we are baptized, the divine intervention of God, the LIGHT of Christ, washes away the original sin, and we are again one with God and free to live our lives the way we want to, choosing the path to righteousness(RIGHT) throw the light of christ, or the WRONG through the darkness of the Fallen One.

Now with paranormal investigating, one can almost certainly pick-up a "dark" spirit basically by the feel of the enviornment around you. You can sometimes feel the pain and emotion of that individual or the darkness of an event that might have taken place, likewise you can also feel a "good" spirit by feeling at ease and if it communicates openly to you.

On a final note, as you made reference to evil spirits causing mental and physical harm. That sometimes does happen. If it is truely a demonic presence it will torment you on all levels. It's main purpose is to break you down so much inside and out that it is trying to get you to turn your back from the LIGHT, God, and religion and choose he DARKNESS. It is at that time you must choose your path. As hard as it is to say this, but you must at that time fight off the "inner demons" you refer to, and it is the ultimate test of your STRENGTH, your WILL(Free Will I mentioned earlier) and most of all your FAITH.

It is possible to break a demons hold on a person. When in a situation faced with demonic activity, for examply exorcisms, it is the strength of the people in the room, the strength of the priest/exorcist(In Roman Catholic faith), and most of all, the one being possessed that will break the hold and cast the demon back into the lake of fire. You see, a lot of this ties back into religion in some way shape or form. Just food for thought. Anyone care to make a point here?