Imagine a servant sent on an errand. He finds it impossible to execute the instructions received from his master. That ends it, if he is deficient in intelligence. No instructions have been given for this case; thus there is nothing to do but to return home. But the thought of an intelligent servant is more comprehensive. He recalls his master’s situation and analogous cases; the probable purpose of the master’s order; other possibilities of realizing the same end. Thus he succeeds perhaps in reconstructing the totality of the conditions which led his master to send him, and in meeting these conditions. Take another example. Of several physicians, all but one are mistaken in the diagnosis of a case. Why do they differ? Every disease is characterized by a multitude of symptoms. Some of them are obvious, so that no one can fail to notice them: the complaints of the patient. Others are more hidden, but no less important. The physician must search for them. Each symptom, for example, fever, lack of appetite, dizziness, megalomania, may appear in very different diseases. A definite group of symptoms in definite degrees of intensity is characteristic of a particular disease. Two conditions, therefore, must be fulfilled to make a correct diagnosis. The symptoms which are hidden must be called up by those which are obvious, so that the physician can search for them and determine whether they are present or absent; for without first thinking of them he cannot search for them. Secondly, the thought of the present and absent symptoms must reproduce the idea of the disease which is characterized by the presence or absence of just these symptoms. This reproduction is possible only in a mind in which all these ideas are very closely connected, forming a well-organized system. Where this is not the case, the less obvious symptoms cannot influence the decision, and the correctness of the diagnosis becomes a matter of chance.