The profile of Caesar Augustus floats on the centre of my computer screen. The emperor looks youthful, slender: this coin was struck in the first years of his reign. And surely some of the power of the artifact flows from its surrounding context. The end of the Roman Civil War. The slide from Republic into Empire. Cicero's dream that Jupiter himself chose the young ruler. Suetonius, eighty years later, telling how the emperor's own wife helped to select virgins for his imperial enjoyment. The coloured glass glinting from mosaics in those élite chambers. The customary platters of flamingo breast, spiced with cumin and mint. It seems that history has cast a force field around this little slice of silver.

But for moments, peering at the screen, I suspect the coin holds my eye precisely because it's cut off from history. It has escaped. This feeling intensifies when I look at the reverse side, which shows the image of Capricorn balancing the globe between his hooves. He seems a being unto himself, his reproduction in metal one instance of human artfulness winking across millennia. I know that the coin maker began by holding a blank silver disc with his tongs. He kept it in the fire until the silver was red and malleable. He placed it between the dies he had engraved, then struck the dies with a hammer, opened them and dropped the coin in the sizzling water.

Neither of these two ways of seeing ...