For Charles, sine qua non

The deck of the French ship was slippery with blood, heaving in the choppy sea; a stroke might as easily bring down the man making it as the intended target.

In his heart he shared the simplicity of Temeraire's view of the matter, and he fell asleep almost at once in the security of the slow, deep rushing of Temeraire's heartbeat, so very much like the endless sound of the sea.