I've been using the word "world" a lot in this blog and even more on the thesis essay. I'm going to borrow some more of Joseph Pieper's language to explain what I mean by "world" (this is also very Aristotlelian).
When thinking about human beings, it's important to distinguish the world from the environment.
The environment, which human beings also live in, describes that place where animals live and interact. It is governed by the natural laws, and our interactions on this level (because we are, after all, wholly animal) are instinctive.
The world, which is reserved for rational beings (human beings, angels, God), is a new level of interaction. If you live in the world, you have a conscious relation with all things. That is, because you have the ability to know things and know yourself, you inhabit the world.
Irrational animals, in traditional philosophy and even in almost all philosophies, do not inhabit the world. They stop at the level of instinct and natural laws. They don't have a personal relationship with other animals, nor are they conscious of having any relation at all with the world. They simply live, survive, and die. But we are aware that we live, survive, suffer, laugh, and die.
This is not to say that irrational animals are not in some sense worthy of our respect (though the nature of that respect is worthy of thousands of blog entries). It is simply to make a distinction between the world we live in by virtue of our desire and capacity to know and the environment we cope with on the level of our animal nature.
Being a thinking being is not all fun and games and mastery of the universe. Obviously, it is sometimes rather a bore to be conscious that I will suffer and die or that the things I hold dear at this moment may be gone the next. That is living in the world, beyond the immediate demands of the environment. But I also am able to contemplate metaphors ("God is like those mountains") or compose songs; I can anticipate with joy my child's next accomplishment; I can work to grow in virtue and wonder why it is that men can't find what they're looking for.
Dear Pascal wrote it out so beautifully: "Man is only a reed, the weakest in nature, but he is a thinking reed. There is no need for the whole universe to take up arms to crush him: a vapor, a drop of water is enough to kill him. But even if the whole universe were to crush him, man would still be nobler than his slayer, because he knows that he is dying and the advantage the universe has over him. The universe knows none of this. Thus, all our dignity consists in thought... Let us then strive to think well; that is the basic principle of morality." (Pensee 200, L)