Kant (forms of thought):

the most basic & first forms of thought were defined by Kant as being a priori ('from the earlier'), as knowledge independent of experience.  It was a category or form of thought.  This idea was popularized by his Critique of Pure Reason (1781).

 

scientific psychology: * see EndNotes<A>

aka experimental psychology; work done by those who apply experimental methods to psychological study & the processes that underlie it; it employs human participants & animal subjects to study a great many topics, including sensation & perception, memory, cognition, learning, motivation, emotion; developmental processes, social psychology.  Germany & German scientists were leading players in early experimental psychology.  Spengler would have known of this study and derides it as a science of cause-effect.

 

ab initio:

Latin, from the beginning