 Wright, then, regarded the nature of verification as evident and without problems of interpretation. Verification was part of the solution to the problems that beset theory-making and explanation, e.g., the competing claims about what theoretical entities exist, and what factors should militate for or against acceptance of any theory or cosmology. Asserting the priority of verification as the judge of theory, Wright said that discussion of the origin of theories or any claim for their a priori character is of no moment in science, “which maintains strict neutrality toward all philosophical systems” (PD 47). He said that the only difference between theories and facts is that theories are more complex and less directly testable (PD 44). Unlike later logical positivists, however, Wright did not hold that terms or descriptions for theoretical entities were meaningless or to be resolved only into propositions stating their verifiable consequences. The unobservables postulated by science are “for the purpose of giving a material or visual basis to the phenomena and empirical laws of life in general” (PD 164–65), and some of them will be proven to exist. In this regard, he likened Darwin's gemmule theory to Newton's corpuscular theory of light and the molecular theory of matter. In alluding to the difficulty of representing the extremely small size of molecules as measured by Thomson, Wright said: