Descriptive theories of behaviour may allow for the scientific measurement of psychological attributes

Trendler (2009) argued that psychological attributes cannot be measured, as the experimental manipulation and control necessary for the application of measurement theory cannot be achieved. It is argued that Trendler's conclusion ignored deeper issues. The scientific measurement of psychological attributes depends not only upon adequate stimulus control, but also upon descriptive theories of psychological systems and the demonstration of pure differences in degree (magnitude) within attributes hypothesized to be quantitative. For some classes of stimuli, where descriptive theories of the response process exist and where quantitative features in the stimuli themselves can be empirically manipulated, the demonstration of pure differences in degree is plausible and the scientific measurement of the relevant attributes credible. Where attribute differences between stimuli have identified qualitative causes, stimuli cannot be engineered to produce equivalent magnitudes of the relevant attribute. Here is where Trendler's Millean Quantity Objection has force.