CHOOSING PHYSIOLOGICAL VARIABLES 435 



constants has never been more than that of parameters in equa- 

 tions. It may be satisfying to suppose that they are more, and it 

 gives an impetus for experimental tests if those suppositions are 

 followed through. But the satisfaction is no part of the structure 

 of ascertainable facts and relations, nor is the realization of predic- 

 tions a proof of their theoretical basis. For there are usually more 

 possibilities than have been imagined. The satisfaction gained by 

 one experimenter may be unwelcome to another; probably what- 

 ever is permanent in science will be limited to the "purely statis- 

 tical." Mathematical language has for most men the fewest im- 

 plied notions and fancies. 



Some physiologists suppose that they can distinguish a ''di- 

 rect" connection between two phenomena from an "indirect" or 

 incidental one; a "primary" or "fundamental" or "essential" 

 factor or nexus from a "secondary" one. But no rule is given for 

 making this distinction. Evidently a "direct" nexus is one that 

 agrees with certain hypotheses, or with certain types of hypotheses. 

 Perhaps expertness is even signalized by insensitiveness towards 

 the assumptions implicit in its own field. 



Some do not think phenomena exist until a plausible "theory" 

 (of physicochemical mechanism) is found for them. Thus, they 

 find it unbelievable that dogs drink water in amounts approxi- 

 mately equal to their deficits of water, or that rats select foods con- 

 taining vitamin A when vitamin A is depleted, even though mea- 

 surements have been reliably presented. They say, before we 

 accept those correlations show us what physical and anatomical 

 features are concerned, "how" the animals make the choices. 

 They limit their nexuses to particular sorts, and quantitative rela- 

 tions seem to them to leave these phenomena as "isolated facts" 

 of doubtful existence. 



I hope it is understood that the broad statements of this chapter 

 are simply the projections of experience and inference. The 

 rigorous inductive procedures of previous chapters have now been 

 supplemented by extrapolations whose force is of the same magni- 

 tude as those of persons holding other opinions. These extra- 

 polations issue from experiences with those procedures that have 

 been emphasized in this investigation. 



So, present day physiology is exposed to the distinct, not 

 mutually exclusive, possibilities; (a) That several methods of dis- 

 covery and of investigating relations among phenomena will be 



