Mind acting IN World
One theme of the essay is different types of definitions used in different disciplines.
Psychology emphasizes "operational definitions" in which the state of mind is inferred from behaviors in the objective world. Measurements may be physiological, as in heart rate and skin conductance. Other measurements are of the self-report nature, such as endorsing statements as true or false of one's self, an example of which is the ancient instrument MMPI. Self-report inventories are validated by factor analysis.
Many infinite subjectives and one finite objective. Correlating the subjective and the objective. Inferences can only be made of external expressions.