I had this discussion with some people a few years back, though many didn't grasp the concept. With them being 'into' complementary therapies and the like, they would often excuse the "bad" or "evil" things that happened to people as necessary or part of spiritual life lessons (the same was also raised on the question of why God let's "bad" things happen) saying things like "you can't have good without evil, and therefore evil must exist in this world".
My argument was that "good" and "bad"/"evil" are merely labels created by the human mind / society (community mind), and what some people see as bad or evil is seen as good in other cultures and visa versa. These labels are created by people 'need' to differentiate between different things; to categorise things; to create a quality between things. As Robert M Persig talked about in his book Zen and the Art of Motorcycle Maintenance, where he investigates subjective and objective quality, if there was no such thing as quality, then a blank wall would look just as appealing as a wall with a picture on it.
Whilst many didn't/don't agree with me (that's their choice

), it IS possible to have good without bad/evil i.e. everything good. The difference would be that we would not be able to quantify the good as we would have nothing to compare it against; nothing to give it the "good" quality. Just because something can't be quantified does not mean that it doesn't exist.
Likewise there is beauty in everything. One person may look at an object and see it as beautiful, whereas another may look at it and find it disgusting. The object cannot be both beautiful and disgusting and therefore the quantification of those people is purely subjective; the fact that beauty has been recognised in the object means that it does have beauty (and by the same token it does have disgust), but in truth it is neither of those things, as it simply IS.
In Buddhist terms they talk of "emptiness" which is often something that is not explained well or understood well, and probably that comes from the fact it's labelled "emptiness" whereas it relates closely to interdependence (or Inter-Being as Thich Nhat Hahn calls it). What it really refers to is "empty of being seperate". A good example to demonstrate this is a glass. The glass is not just a glass, but is made from the silica components, and would not exist without having had those components created by the earth, using elements of the sun, earth, water as well as the people involved in creating it in it's current shape and form to deliver it, and the things that created those people (their mother and father and their mothers and fathers, their teachers, the food and the suppliers of their food etc. etc. etc.) and so it goes on. This is interdependance, and from it we can see that the glass contains everything in the universe. It is not seperate from anything. We can look at the glass filled with water and say the glass if full of water, but if we look at the glass without water we say it is empty, but empty of what? Nothing can be empty, unless it is empty of
something. The glass is empty of water, but still contains everything. This is why emptiness is synonymous with Oneness.... hence, you can seem to take the evil out of something to leave goodness, but the goodness is everything anyway, including the evilness. Both exist, both are empty and both are merely labels created by us to discriminate in our attachment to quality. Let it go!
So yes, we are all already enlightened, we are all already complete of knowledge, we are all emptiness. We simply cover these things up with our ego monkey minds.
Hugs
Giles