 The preservationist conception is reflected in the empiricist theory, which was influential in the first half of the twentieth century and is thus the natural starting point for a review of theories of remembering. The most influential theories in the second half of the twentieth century were the epistemic theory and the causal theory, which likewise reflect the preservationist conception, with the causal theory gradually eclipsing the epistemic theory. In the early years of the twenty-first century, the causal theory has been challenged by new simulation theories, which adopt a thoroughly generationist conception of memory. The remainder of this section reviews each of these theories in turn.