Why It's Hard to Define Simple Terms

Augustine famously asked:

For what is time? Who can easily and briefly explain it? Who even in thought can comprehend it, even to the pronouncing of a word concerning it? But what in speaking do we refer to more familiarly and knowingly than time? And certainly we understand when we speak of it; we understand also when we hear it spoken of by another. What, then, is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not. (Confessions, 11, XIV, 17; my emphasis)

Why are basic things difficult to define? Tom Morris, in his fine book Our Idea of God: An Introduction to Philosophical Theology, has a helpful word on this:

The idea of a power is a very basic, fundamental idea. It is such a basic idea that it is very difficult to analyze or explain, since analysis and explanation typically break up the complex into the simple, or illuminate the unfamiliar by reference to the familiar. We normally give an account of one idea by explaining it in terms of more basic ideas. But the idea of a power is so basic in our conceptualization of the world that it is hard to find much to say in elucidation of it. (p. 69)