Explanation is possible by virtue of the structural relation of concretion to abstraction. Explanation is possible in a symbolic system wherever it can be shown that two symbols are related as implicans to implicate. Upon the basis of such a relation it is possible to divide the elements of integrated symbolic systems into two groups, the derived elements and the underived elements, or derived propositions and underived propositions. The suppositional symbol will be a word suchas “molecule,” “electron,” “perfect lever,” “point,” “ether,” or an algebraic symbol for a quantity, which designates any obscurely given occurrent operationally defined and employed as a symbol of explanation. A critical positivism will reconcile the two positions by an analysis of operational routes. This will reveal the norms for thinking in terms of suppositional symbols, and will thus afford a criterion by means of which adequate suppositional symbols may be distinguished from inadequate ones.