The author of a textbook must employ a scientific and dynamic heuristic

Now as the statistical approach differs from the descriptive, the analytic differs from both. Out of endless classificatory possibilities it selects not the one sanctioned by ordinary speech nor again the one sanctioned by facility of measurement but the one that most rapidly yields terms which can be defined by the functional interrelations in which they stand. [CWL 21, 112]

Let us say, then, that for every basic insight there is a circle of terms and relations, such that the terms fix the relations, the relations fix the terms, and the insight fixes both.  If one grasps the necessary and sufficient conditions for the perfect roundness of this imagined plane curve, then one grasps not only the circle but also the point, the line, the circumference, the radii, the plane, and equality.  All the concepts tumble out together, because all are needed to express adequately a single insight.  All are coherent, for coherence basically means that all hang together from a single insight. CWL 3, 12/

See Scientific Explanation

See Postulating without Explaining

See Precise Analytical Distinctions and Concepts