Hey all, I studied Counterpoint a while back and have recently felt like refreshing my memory with some counterpoint compositions.
There's a gap in my memory though, and nothing in my notes, regarding what motions are available going from or to a dissonance?
I'm hoping I just read my sources wrong and there's an easy explanation. I'm assuming it's not a flat-out "you can't use dissonances" as that would mean we're limited to melodies containing only Roots, 3rds, 5ths and 6ths. And that would suck.

Do dissonances count as imperfect?
are you talking about strict counterpoint? if so, which species?
In first species a melody will be harmonized in only 3rds, 6ths and perfect 5ths (and the odd octave).

Once you get into other species, other dissonances are allowed.

2nd species - unaccented passing notes
3rd species - Passing, accented passing, cambiatas, auxiliary
4th species - suspensions
The species of which you are focusing on governs the type of harmony available to you.

If i remember correctly from my notes, a summary of harmonic restrictions would be something like,

1st species:
Melody - Stepwise melodic movement, 3rd, 8ve, per 4th/5th intervals, minor 6th upward movement only.
Harmony - Consonant intervals only, unison at beginning and end only, 3rd (M/m), Per 5th, 6th (M/m), 8ve, 10th, 12th (Max limit)

2nd species: 2:1 cantus firmus speed ratio
Inclusion of passing notes, unaccented passing notes can be dischordant provided that it moves stepwise down between consonant notes a 3rd apart.

3rd (1+2) species: Suspensions of the 7th, 4th and 2nd against the consonant notes.

They're the easiest ones to explain via words, 4th and 5th species looks more into melodic and rhythmic syncopation and more elaborate forms of suspensions.

...as far as i'm aware
^ There's no suspensions in third species, and in 4th you can have both dissonant and consonant suspensions.