Modernity from the time of Descartes experienced what Kant would call antinomies (unresolvable dilemmas) with regard to questions of freedom and determinism. These were not eternally given, but resulted from specific, contingent historical developments — not only emerging modern science, but also some very specific features carried forward from medieval European theological controversies. (See Errors of the Philosophers; Pseudo-Dionysius on the Soul; God and the Soul; Mind Without Mentalism.)
Since the early modern period, many philosophers have struggled to formulate their own incompatible ways of trying to assert both billiard-ball causality and a hyper-strong concept of personal identity to which something like voluntaristic free will was attributed. Even Kant and Hegel used a good deal of socially acceptable voluntaristic-sounding rhetoric that was at odds with their more careful arguments. (In a real world where audiences have prejudices, philosophers who want to be heard have to be careful to gauge their audience, and pick their battles wisely.)
In this context, Spinoza stands apart from the rest. Like the others, he wanted to assert modern-style univocal causality, but the kind of freedom of reason he argued for in Book V of the Ethics did not presuppose the hyper-strong personal identity and free will others wanted to assume. He emphasized the relational character of all determination, and famously mounted a head-on critique of free will.
Ethically, Spinoza was profoundly committed to reason and an immanent understanding of nature and society. He published the first critical textual analysis of the Old Testament, and was among the first open advocates of free speech. His work was a major inspiration to the left wing of the Enlightenment that gave us the ideal of democracy. The Left Hegelian Ludwig Feuerbach called him “the Moses of the freethinkers”.
Among philosophers, he also stands out for giving unprecedented attention to emotions and their interaction with reason. He is particularly concerned with the harmfulness of sad passions, and recommends that we use joyful passions as well as reason to help free ourselves from the grip of the sad passions.
Spinoza rejected the theologized Aristotle, and knew no other version. In spite of this, I think Aristotle would agree that Spinoza’s arguments against free will by no means rule out Aristotle’s moderate conception of deliberation and choice. The problem has been that many medieval and modern authors felt it necessary to defend what were actually unnecessarily extreme versions of freedom. (See also Ends.)
While praising Spinoza’s monism, Hegel alleged that Spinoza’s thought led to an “oriental” dissolution of personality into the One, and things like that. We might say he should have paid more attention to Spinoza on the freedom of reason, but Terry Pinkard’s good biography of Hegel attests that he already worried about police scrutiny of his views, and a failure to substantially criticize Spinoza would have placed him in the company of the extreme left. In the literature on Hegel, Hegel’s references to Spinoza are too often simply accepted at face value, which is a great injustice.
To the extent that Spinoza has limits in comparison with Aristotle, Kant, or Hegel, it is perhaps in a tendency to simplify; and in his relatively univocal and static, basically one-level (albeit highly relational) conception of nature.
Pierre Macherey’s untranslated five-volume, line-by-line French commentary on the fine inferential structure of Spinoza’s Ethics is, I think, the best discussion of Spinoza in any language. A smaller work of Macherey, Spinoza or Hegel, has been translated to English, and develops many points I would wish to make.
In a very different vein, the 1934 classic study by Harry Austryn Wolfson pointed out many hidden allusions in Spinoza to arguments and positions from the Hebrew tradition of medieval philosophy. Wolfson called Spinoza “the last of the medievals and the first of the moderns”.
Medieval connections also figure in Gilles Deleuze’s 1968 thesis Spinoza: Expressionism in Philosophy, which I find to be very uneven. On the one hand, in a long digression expounding the “expression” theme, Deleuze had interesting things to say about relations between implication and explication, or folding and unfolding, in relation to Spinoza and late neoplatonism. On the other, he made what I now think was terribly wrong use of the “univocity of being” thesis of the Latin theologian Duns Scotus, with whom he wanted to closely link Spinoza. I believe both Spinoza and Scotus would have been appalled by this suggestion.
The historian Jonathan Israel has documented the large importance of the Spinozist movement in the Enlightenment. The important scholar of German Idealism, Frederick Beiser, gave a fascinating account of the mostly very hostile German philosophical reception of Spinoza around the time of Kant in part of his first book The Fate of Reason (1987).
Brandom characterized Spinoza as a proto-inferentialist in Tales of the Mighty Dead.
I like the fact that Spinoza’s main philosophical work was simply called Ethics. Spinoza sought to develop a truly philosophical ethics, incorporating a wide range of meta-ethical considerations.