rationalia

[…]; and afterwards, by exhibiting all the curious and leading proceſſes, with particular obſervations on each, explaining the rationalia of the proceedings, and pointing out the application of them to practice in medicine, metallurgy, and other arts, and to the improvement of natural knowledge in general;[…]

1760, An Essay on the Medicinal Nature of Hemlock

Not a single reason is given in the work for the propriety of any one construction: nor are any observations inserted that might point out the advantages or disadvantages of the component parts of the methods delivered in it or that are calculated to lead young minds to study the rationalia of the profession.

1808, The Monthly Review, or Literary Journal, page 264

This work, “written in the hope of providing the intelligent gardener, and the scientific amateur, correctly, with the rationalia of the more important operations of horticulture,” has supplied a very important desideratum, and is deemed indispensable to every gardener and amateur cultivator in Great Britain.

1841, The American Journal of Science and Arts, volume XLI, New Haven: Hamlen, B. L., page 210

This is but a brief outline of what the principles are upon which the common operations of the Fruit Garden depend; yet it is hoped that it may not be without its use in calling attention to the rationalia of what may seem extremely simple and well-understood practices, but which are undoubtedly neither so perfect, nor generally so skilfully performed, as to be incapable of amendment.

1852, A Guide to the Orchard and Fruit Garden; Or an Account of the Most Valuable Fruits Cultivated in Great Britain, New York: Saxton, C. M., page 338

“[…]The public mind is, as a rule, not sufficiently acquainted with the rationalia involved to be capable of self-protection, or to be able to distinguish, as scientific men generally do, what has been definitely discovered from what has in the absence of discovery been plausibly suggested.”

1871, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, volume XLII, pages 69–70