“So far as we understand it,” she began, “every solid and every gas in the cosmos is built from luxagens. From a cosmic perspective all luxagens are identical, but because their histories through four-space are marked with a kind of arrow—Nereo’s arrow—we can distinguish between those luxagens whose arrows point toward our future and those whose arrows point toward our past. By convention, we call the first kind ‘positive’ and the second ‘negative’.
