In my first piece on the book, I commented on the way Spinoza's philosophy seems closely allied with classical liberalism. Spinoza did not believe in a transcendent God, and so he looked for a way to base ideas of morality, purpose and freedom in nature.
He did so by identifying the "conatus" as a central principle: the striving of all things to preserve their own self. Self-preservation is therefore not, in Spinoza's philosophy, an important but mundane aspect of existence; instead, acting according to our self-interest to preserve our being is the key aspect of morality; and the absence of external constraints in pursuit of this self-interest is the definition of freedom.
Here is how it is put in Stewart's book:
Spinoza, like most modern theorists, grounds the legitimacy of political authority in the self-interest of individuals. He argues not only that everyone, and every thing, for that matter, is driven by self-interest but that they ought to be as well. "The more every man endeavours and is able to seek his own advantage, the more he is endowed with virtue," he says in the Ethics. "To act in absolute conformity with virtue is nothing else in us but to act, to live, to preserve one's own being (these three mean the same) under the guidance of reason on the basis of seeking one's advantage." (pp.101-102)
For Spinoza, man is defined by his desires:
"desire is the essence of man," as Spinoza puts it. To be clear: this desire is fundamentally self-centred. (p.175)
Leibniz was initially attracted to Spinoza's philosophy, but he recoiled at some of its implications, especially the denial of an immaterial soul. He prophesied that Spinoza's materialistic philosophy would undermine Western culture and civilisation.
Leibniz attempted to establish an alternative philosophy, one in which there was no single material substance in the world (a world soul) from which everything else emanated, but instead one built on a variety of substances called monads.
A difficulty in Leibniz's philosophy is that the monads are thought to be set in a pre-ordained harmony with each other - which then makes Leibniz something of a determinist (p.285). Another difficulty is that Leibniz thought that the purpose of life was for each monad to become autonomous, the better to realise its own self. Autonomy was achieved by the correct use of reason, by which monads were delivered from the influence of the passions (p.291).
Conclusions? Stewart sees Spinoza as carrying the day, but at a considerable cost:
Leibniz, perhaps alone with Spinoza, grasped the general direction of modern history. But, unlike his eerily self-sufficient rival, he had a far greater concern with the price that humanity would have to pay for its own progress.
He understood that even as science tells us more and more about what everything is, it seems to tell us less and less why; that even as technology reveals utility in all things, it seems to find purpose in nothing; that as humanity extends its powers without limit, it loses its faith in the value of the same beings who exercise that power; and that, in making self-interest the foundation of society, modern humankind finds itself pining for the transcendent goals that give life any interest at all. (pp. 254-255)
I'll finish with a rough conclusion of my own. Spinoza saw himself as a revolutionary who aimed to overthrow Christian theology and replace it with another, "naturalistic" theology.
His efforts to create this new theology led him to certain principles. This included certain ideas about life and liberty, namely that virtue means acting according to self-interest to preserve our being and that freedom is an absence of external constraints in the pursuit of this aim.
To the extent that these ideas were brought into classical liberalism through the influence of Spinoza, classical liberalism can be seen as the product of an attempt to replace Christian theology with a theology based on a concept of an immanent God.
But, I admit, I don't know to what extent the existence of such ideas within classical liberalism can be attributed to the influence of Spinoza.