Take the principle of mathematics concerning addition. We know that this is a true principle because we can confirm that 2 plus 2 make 4 in real life by adding 2 sets of objects together.
Hence, applied mathematics gives us a reason to believe that a principle of pure mathematics is true. However, the principle of pure mathematics is a prerequisite for the work in applied mathematics because if we are to do any empirical testing, we must first have a clear idea of what it is that we are testing.
Thus, before we go on to look for Fi or Ti in people, we must first have an idea of what Fi or Ti is. This justifies the precedence of pure typology over applied typology.
How would this work? Suppose I make a hypothesis that there is a Thinking function, and such a function is responsible for logical reasoning and it contraposes with subjective value judgments. We go on to do empirical testing. We study the works of philosophers, writers and scientists to see if we observe such tendencies. If we do, than we can conclude that we have discovered a typological principle.