[…] [the effect of conceptual association] stands in the foreground of neogrammarian studies. It is cited as the antithesis to the regularity of sound laws, as the "psychological" factor in contrast to the "physiological" factor [...] First of all there is the possibility of subordination: one factor is the constitutive or normal one. The other is the disruptive or anomalous one. The latter was then equated with the psychological factor. However, if we rely upon exterior appearance in this process, we can ask ourselves whether or not there could be evidence of cases [...] where the isolated effects of sound laws seem to disrupt great analogical groups [...] General reservations about conceding that capricious factors interfere with a fixed order are added to such special observations. Thus we are forced from every side to recognize that regularity is inherent in the psychological as well as the physiological linguistic principle. In other words, we are obliged to coordinate the two. The peripheries of their spheres of influence intersect each other at many points. Which one prevails over the other depends upon the immediate circumstances in every given case. (p. 43)
- Schuchardt (1972)

Analogical exceptions are impossible whenever the purely physiological cause of a sound substitution cannot be doubted, a cause such as a peculiar formation, a natural or artificial defect of the vocal organs. Wherever we do find such exceptions we have to relinquish the thought of purely physiological causation. The psychological nature of one of the interacting factors gives direct evidence of the identical nature of the other […] Thus the antithesis melts away before our eyes and the problematic character of the external relationship between the two factors [physiological factor and psychological factor] becomes clear when we understand their internal relationship correctly. (p. 44)
- Schuchardt (1972)