On a more serious note than my last effort to link these words;
After a few minutes of thought I feel that what links them together for me, is that they are all dependent on something else for them to exist.
For example;
Gravity; as they tell us, is non existent without a piece of matter above an indiscernible mass. Only through the existence of this matter, can gravity then exist.
Economics; Without people, there would be no need for trade, meaning there would be no such thing as economics.
Life; All life is dependent on reproduction, and so we would not exist had it not been for another life granting us our existence. If our ancestors did not exist, neither would we.
Just a thought. I find this interesting so I will smoke a joint and get back to you on this one.
dang it...you beat me to it!!!