I distinguish two conceptions of bias. First, bias can be understood as unwanted error that hampers cognition and should be eliminated. Second, it can be understood as functional; it is necessary for cognition. The dominant interpretation in the social sciences is the former, which links biases to prejudice or cognitive errors. It is based on norms such as consistency, coherence, and social norms. I provide examples for functional biases that protect individuals and enable perception. Finally, I provide a general perspective on the apparent contradiction between bias as error and functional bias, based on the distinction between small and large worlds. My hypothesis is that bias is detrimental and without any function in small worlds. In all other situations that involve a substantial degree of uncertainty, bias is likely to be necessary and functional, or even rational.