A Excerpt from Essay on the True Art of Playing Keyboard Instruments

(...) In order to control all shades from pianissimo to fortissimo the keys must be gripped firmly and with strength. However, they must not be flogged; but on the other hand there must not be too much restraint. It is not possible to describe the contexts appropriate to the forte or piano because for every case covered by even the best rule there will be an exception. The particular effect of these shadings depends on the passage, its context, and the composer, who may introduce either a forte or a piano at a given place for equally convincing reasons. In fact, composite passages, including their consonances and dissonances, may be marked first forte and, later, piano. This is a customary procedure with both repetitions and sequences, particularly when the accompaniment is modified. But in general it can be said that dissonances are played loudly and consonances softly, since the former rouse our emotions and the latter quiet them (exercise a). An exceptional turn of a melody which is designed to create a violent affect must be played loudly. So-called deceptive progressions are also brought out markedly to complement their function (b). A noteworthy rule which is not without foundation is that all tones of a melody which lie outside the key may well be emphasized regardless of whether they form consonances or dissonances and those which lie within the key may be effectively performed piano, again regardless of their consonance or dissonance (c).
(...) If the Lessons are played on a harpsichord with two manuals, only one manual should be used to play detailed changes of forte and piano. It is only when entire passages are differentiated by contrasting shades that a transfer may be made. This problem does not exist at the clavichord, for on it all varieties of loud and soft can be expressed with an almost unrivaled clarity and purity. A loud, boisterous accompaniment must always be balanced by a stronger melodic touch.
(This is the standard English translation by William J Mitchell, Essay on the True Art of Playing Keyboard Instruments. New York: Norton, 1949. Emphases mine.)