I remember thinking about this when the semantic web was first being discussed. If you think of it from the perceptive of a child, your first 'foundational' words are learned though direct experience. Then while you continue to learn words this way, we can also use those words we 'know' to define secondary or tertiary terms that we have no direct experience of. I'd like to see a graph like this with someones take on the minimum number of necessary foundational words and how that graph would look.