All 5 Uses of

rational
in
The Magic Mountain

He had had enough of the cooking here, he said, everyone up here had, and it was customary to disparage the food, because when you had to sit up here forever and a day ...But he did enjoy drinking, taking to the wine with something like abandon; and while carefully avoiding all sentiment-laden phrases, he repeatedly expressed his satisfaction that at last someone was here with whom it was possible to have a rational conversation.†
Chpt 1.3

rational = reasonable

That history had in fact been one of the progressive application of human reason, by which, on the basis of purely rational concerns, God had gradually been removed from the administration of justice.†
Chpt 6.6 *

It was, moreover, the period when Rosicrucianism infiltrated the lodges—a very strange brotherhood, which, you should note, united the purely rational, sociopolitical goals of improving the world and making people happy with a curious affinity for the occult sciences of the East, for Indian and Arabic wisdom and magical knowledge of nature.†
Chpt 6.8

The reemergence of the Templars had meant nothing less than the establishment of such connections; it had introduced the ferment of irrationality into an intellectual world concerned with rational, practical social improvement.†
Chpt 6.8

Definitions:

(1) (rational as in: rational behavior)
reasonable, able to think clearly, or based on logic rather than emotion
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More rarely, rational numbers in mathematics are numbers that can be expressed as a fraction consisting of only an integer in the numerator and denominator. For example, 1/3 is a rational number while pi is an irrational number.