My smile has been my ticket to the world. Smiling releases the same feel-good hormones you get jogging. Caring for your lips and gums is important. I brush my teeth morning and night, alternating toothpaste brands. In addition to flossing, I use a Water Pik to massage my gums and remove food particles.

As in nature, all is ebb and tide, all is wave motion, so it seems that in all branches of industry, alternating currents - electric wave motion - will have the sway.

We say that a human being is a person and a distinctive, fixed self with a name and a life. He has an identity. But what is this self really made of, except from the basic elements such as hydrogen, carbon, oxygen, phosphorus etc. and their subatomic particles? If a person is a specific, static, unchanged entity and existence, then what if an accident or a disease completely alters his body features? What if fear or madness changes his thoughts and perceptions? If dementia takes away his memories, or if drugs alter his emotions? And what if life circumstances, good or bad luck, modify his motives, his plans and his desires? Is it still the person we say he is? Or is selfhood a ghost, a useful fiction of the brain? An ever-shifting kaleidoscope of thoughts, feelings and perceptions? Flashes of hopes and desires? A bundle of alternating opinions and ideologies, of conflicting instincts and urges? If we take away all these from him, what would be left behind? If every drop of the ocean evaporates, is not the whole ocean gone? The immutable selfhood is a very old illusion and the last of illusions we 're going to abandon; if we ever will?

Walking causes a repetitive, spontaneous poetry to rise naturally to the lips, words as simple as the sound of footsteps on the road. There also seems to be an echo of walking in the practice of two choruses singing a psalm in alternate verses, each on a single note, a practice that makes it possible to chant and listen by turns. Its main effect is one of repetition and alternation that St Ambrose compared to the sound of the sea: when a gentle surf is breaking quietly on the shore the regularity of the sound doesn't break the silence, but structures it and renders it audible. Psalmody in the same way, in the to-and-fro of alternating responses, produces (Ambrose said) a happy tranquillity in the soul. The echoing chants, the ebb and flow of waves recall the alternating movement of walking legs: not to shatter but to make the world's presence palpable and keep time with it. And just as Claudel said that sound renders silence accessible and useful, it ought to be said that walking renders presence accessible and useful.