.. _DefSecondOrderSpec: Definition of Term: Second-order belief specification ===================================================== A belief specification for a collection of uncertain quantities is described as *second order* when the beliefs are expressed solely in terms of expectations and variances for every uncertain quantity, and covariances between every pair of quantities. This differs from a fully probabilistic specification which requires a joint probability distribution over all unknown quantities. The second-order specification is the basis for :ref:`Bayes Linear` methods.