Spinoza took emotions more seriously than any philosopher before him.
“Most of those who have written about the Affects, and men’s way of living, seem to treat, not of natural things, which follow the common laws of nature, but of things which are outside nature. Indeed they seem to conceive man in nature as a dominion within a dominion. For they believe that man disturbs, rather than follows, the order of nature, that he has absolute power over his actions, and that he is determined only by himself. And they attribute the cause of human impotence, not to the common power of nature, but to I know not what vice of human nature, which they therefore bewail, or laugh at, or disdain, or (as usually happens) curse. And he who knows how to censure more eloquently and cunningly the weakness of the human Mind is held to be Godly.”
“…To them it will doubtless seem strange that I should undertake to treat men’s vices and absurdities in the Geometric style….”
“But my reason is this: nothing happens in nature that can be attributed to any defect in it, for nature is always the same….”
“The Affects, therefore, of hate, anger, envy, etc., considered in themselves, follow from the same necessity and force of nature as the other singular things. And therefore they acknowledge certain causes, through which they are understood, and have certain properties, as worthy of our knowledge as the properties of any other thing” (Ethics, book III, preface, Collected Works vol. 1, Curley trans., pp. 491-492).
“By affect I understand affections of the Body by which the Body’s power of acting is increased or diminished, aided or restrained, and at the same time, the ideas of these affections” (book III, definition 3, p. 493).
“[I]nsofar as it has adequate ideas, [our Mind] necessarily does certain things, and insofar as it has inadequate ideas, it necessarily undergoes certain things” (book III, proposition 1, ibid).
“[T]he Mind and the Body are one and the same thing, which is considered now under the attribute of Thought, now under the attribute of Extension” (book III, proposition 2, scholium, p. 494).
“The Mind, as far as it can, strives to imagine those things that increase or aid the Body’s power of acting” (book III, proposition 12, p. 502).
“Love is nothing but Joy with the accompanying idea of an external cause, and Hate is nothing but Sadness with the accompanying idea of an external cause” (book III, proposition 13, scholium, ibid).
(By no means would I suggest that this is the last word on love; it seems to apply mainly to a lowest common denominator usage of “love” that is not what I normally mean when I use the word.)
“Apart from the Joy and Desire that are passions, there are other affects of Joy and Desire that are related to us insofar as we act” (book III, proposition 58, p. 529).
“Among all the affects that are related to the mind insofar as it acts, there are none that are not related to Joy or Desire” (book III, proposition 59, ibid).
Based on these and similar principles, he develops a sort of physics of the emotions, which on the whole yields surprisingly plausible qualitative predictions of how people will behave in various conditions.