Tristitia est hominis transitio a majore ad minorem perfectionem.
Sadness is the movement of a person from a greater to a lesser perfection.
EXPLICATIO: Dico transitionem. Nam lætitia non est ipsa perfectio. Si enim homo cum perfectione ad quam transit nasceretur, ejusdem absque lætitiæ affectu compos esset; quod clarius apparet ex tristitiæ affectu qui huic est contrarius. Nam quod tristitia in transitione ad minorem perfectionem consistit, non autem in ipsa minore perfectione, nemo negare potest quandoquidem homo eatenus contristari nequit quatenus alicujus perfectionis est particeps. Nec dicere possumus quod tristitia in privatione majoris perfectionis consistat nam privatio nihil est; tristitiæ autem affectus actus est qui propterea nullus alius esse potest quam actus transeundi ad minorem perfectionem hoc est actus quo hominis agendi potentia minuitur vel coercetur (vide scholium propositionis 11 hujus). Cæterum definitiones hilaritatis, titillationis, melancholiæ et doloris omitto quia ad corpus potissimum referuntur et non nisi lætitiæ aut tristitiæ sunt species.
I say movement. For joy is not perfection itself. For if a person were born with perfection to which one moved, one would be in control of the the same thing without the mood of joy; which is more apparent from the mood of sadness which is contrary to this. For no one is able to deny that sadness consists of the movement to a lesser perfection, not moreover in the lesser perfection itself, since a person is not said to be sad insofar as as one participates in some perfection. And we are not able to say that sadness consists of the loss of a greater perfection for loss is nothing; moreover, the affect of sadness is an action which for that reason is to be able to be nothing other than an action of moving to a lesser perfection, that is an action by which the potential power of a person's acting is diminished or restrained (see IIIP11S). The remaining definitions of hilarity, titillation, melancholy and pain are omitted since they refer to the body above all and are only kinds of joy or sadness.
The final sentence of this explanation emphasizes that the focus of The Ethics is on those desires, joys and sadnesses which have a mental component. There are forms of desire, joy and sadness which are only physical, but The Ethics is not a medical textbook. While this disconnection between the mind and body seems to split Spinoza's strict parallelism, that parallelism only applies fully to Deus Sive Natura. In the case of people, Spinoza demonstrates that the consciousness of the mind narrowly selects from the complex workings operating at any point. So, strictly speaking, Spinoza is addressing affects related to consciousness.