We routinely refer to "the unconscious" in a way that suggests we all agree on what it means - but in fact, the unconscious is a highly contested domain. For some, it's a subterranean layer of emotions and desires that operate deep below the rational mind, and that drive our behaviour in unpredictable ways. For others, the unconscious barely exists at all, and only as a metaphor or linguistic device. There's certainly no science of the unconscious, no empirical evidence that might show us what it is or how it works. This week we're diving deep into (or perhaps just skating across the surface of) the unconscious, with the great early 20th century psychonauts Ludwig Wittgenstein and Sigmund Freud as our guides.