All 4 Uses

rational
in
Preface to Lyrical Ballads
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It was published, as an experiment, which, I hoped, might be of some use to ascertain, how far, by fitting to metrical arrangement a selection of the real language of men in a state of vivid sensation, that sort of pleasure and that quantity of pleasure may be imparted, which a Poet may rationally endeavour to impart.†
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The language, too, of these men is adopted (purified indeed from what appear to be its real defects, from all lasting and rational causes of dislike or disgust) because such men hourly communicate with the best objects from which the best part of language is originally derived; and because, from their rank in society and the sameness and narrow circle of their intercourse, being less under the influence of social vanity they convey their feelings and notions in simple and unelaborated expressions.†

rational = reasonable
and, if metre be superadded thereto, I believe that a dissimilitude will be produced altogether sufficient for the gratification of a rational mind.†
Unless therefore we are advocates for that admiration which depends upon ignorance, and that pleasure which arises from hearing what we do not understand, the Poet must descend from this supposed height, and, in order to excite rational sympathy, he must express himself as other men express themselves.†

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)
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.