Jean P. Rameau, Treatise on Harmony (1722), Book II, chap. 19 (trans. Gossett, 152):

"It would seem at first that harmony arises from melody, since the melodies produced by each voice come together to form harmony. It is first necessary, however, to find a course for each voice so that they all harmonize well together. No matter what melodic progression is used for each individual part, the voices will join together to form a good harmony only with great difficulty, if indeed at all, unless the progressions are dictated by the rules of harmony. ... It is harmony then that guides us, and not melody."

Johann Mattheson, Der vollkommene Capellmeister (1739, trans. Harriss), chapter 5.2-6:

"The art of making good melody comprises the most essential thing in music. No one has, to my knowledge, written with purpose and emphasis on melody. Eveyone considers only harmony. ...against all reason it is mentioned that melody springs out of harmony and all the rules of [melody] must be taken from [harmony]. However, melody is in fact nothing other than the origin of true and simple harmony."

Friedrich W. Marpurg, Handbuch bey dem Generalbass (1755, my trans.), Book I, chap. 1:

"Since at least two melodies, a higher and a lower one, are required for harmony, and since harmony does not really exist before we have the two melodies, harmony must arise out of melody. This is Mattheson's opinion. However, if we consider that not even a simply melody can be conceived apart from the harmonic intent of its intervals, then melody must arise from harmony. This is Rameau's opinion. In my opinion, the most certain view is that harmony and melody arise simultaneously."

Johann N. Forkel, Allgemeine Geschichte der Musik, vol. 1 (1788), p. 24 (Bonds, Wordless Rhetoric [1991], 49):

In good musical composition, harmony and melody are as inseparable as the truth of ideas and the correctness of expression are in language. Language is the garb of ideas, just as melody is the garb of harmony. In this respect, one can call harmony a logic of music, for harmony stands in approximately the same relationship to melody as does logic, in language, to expression. ... correct thinking is a prerequisite to learning the correct expression of an idea. And in just this way, experience has truly taught us that no clear, correct, and flowing melody is possible without prior knowledge of harmony. All skilled teachers of composition...have sensed this, on the basis of experience, and they have advised their pupils not to attempt any melodic expression of musical ideas before they have sufficiently sharpened their feeling for the truth and correctness of harmony. [Harmony and melody] mutually elucidate each other, and while no one is capable of providing rules for the crafting of a good, cohesive melody, without deriving such rules from the nature of harmony...no harmonic progression, on the other hand, can be good if it is not at the same time melodic."

Johann P. Kirnberger, The Art of Strict Composition vol. 1 (1771, trans. Beach/Thym, p. 159):

Since complete harmony is in four parts, the harmony in two- and three-part compositions must always be incomplete. Therefore, it is impossible to judge with certainty what must be omitted from the harmony in the various situations that arise until one has a thorough knowledge of four-part composition."