What is a reasonable doubt

Attention is paid here to a definite form of theoretical irrationality, which bears less on the beliefs that it is rational to adopt or maintain than on those it would be better to suspend or reject, namely reasonable doubt. Because one cannot determine with certainty what it would be rational to believe in every circumstance, most notably when, as in the case of a criminal trial, one cannot claim to have a direct or properly reliable access to the truth of the matter it is common to resort to a criterion less demanding than rationality, namely that of reasonableness, which should be considered not only epistemologically, but also politically and socially. In what consists a reasonable doubt, as it is construed in the common law legal tradition, and how is one to decide the threshold above which, in a criminal trial, the jury is allowed to consider that the evidence provided by the Court actually establishes that the defendant is guilty?