Today we tend to think that such diverse entities as facts of nature, mathematical propositions, and plans for action call for different kinds of accounts. Facts of nature require explanations, mathematical propositions demand proof, and plans for action call for justification. Though explaining, proving, and justifying all count as intellectual activities, it would be alien to most modern frameworks to envisage a general scheme within which these activities would be interrelated, and common denominators established. Still, such a program is hardly unreasonable since all three activities involve understanding. One wants to understand nature, one wants to understand mathematical propositions, and one wants to understand plans for action.