The fourth clue is the "clue of beauty":
"If there is no God, and everything in this world is the product of (as Bertran Russell famously puts it) 'an accidental collocation of atoms,' then there is no actual purpose for which we were made--we are accidents. If we are the product of accidental natural forces, than what we call 'beauty' is nothing but a neurological hardwired response to particular data." (Keller, 133)
Really, when we are face to face with great beauty, great art, great music we inescapably feel that there is real meaning to life. Leonard Bernstein says, himself no religious man at all:
"Beethoven ... turned out pieces of breath-taking rightness. Rightness--that's the word! When you get the feeling that whatever note succeeds the last is the only possible note that can rightly happen at the instant, in that context, then chances are you're listening to Beethoven. Melodies, fugues, rhythyms--leave them to the Tchaikovskys and Hindemiths and Ravels. Our boy has the real goods, the stuff from Heaven, the power to make you feel at the finish: Something is right in the world. There is something that checks throughout, that follows its own law consistently: something we can trust, that will never let us down."
Surely anyone that has been enraptured in the love of another can assent to that.
So what? Just because we feel something doesn't make it true! Righto! But i would argue that what is evoked in these experiences is desire or appetite. Augustine reasons that these unfulfilled desires are clues to the reality of God. We may have a desire for a steak, but that doesn't necessarily mean we will get one. However, doesn't the fact that hunger exists point to the reality that the cure for that hunger exists as well?