The necessary knowledge is that of what to observe
…to have a retentive memory, and to proceed by “the book,” are points commonly regarded as the sum total of good playing. But it is in matters beyond the limits of mere rule that the skill of the analyst is evinced. He makes, in silence, a host of observations and inferences. So, perhaps, do his companions; and the difference in the extent of the information obtained lies, not so much in the validity of the inference, as in the quality of the observation. The necessary knowledge is that of what to observe.